The website you mentioned provides a reasonable outline of what is forbidden, however the official references are mentioned at the bottom:
see articles 23 and 49 septies of Urban Police regulations )
The regulations are obviously in Italian, so I will translate them for you.
Article 23
Original:
Other prohibited acts
The whole territory of the Municipality also remains prohibited in a public place or open to the public:
- to clean people, things and animals;
- to throw or drop paper, liquids, dust or other objects;
- to place decorations, festoons, lights, without proper permission;
- to let any kind of animal wander or abandon;
- play any game which may constitute danger or harassment, outside of its intended location;
- play dangerous or harassing sports or recreation;
- place containers or things under public fountains;
- enter lawns, pick flowers, tamper with plants or otherwise damage them; walking on or sitting on flowerbeds, lawns in public parks and gardens and stopping on grassy areas with any type of vehicle;
- climbing trees, poles, gratings, buildings, monuments, public administration lights;
- lying down or climbing on benches;
- to swim or bathe in all canals, canals, St. Mark’s Basin and in any water mirror near inhabited centres;
- to undress and dress on the public street and in places with public view.
In St. Mark’s Square, along the porticoes and steps of the Procuratie Nuove and their continuation to the Napoleonic Wing and the Sansoviniana Bookshop, in the Piazzetta dei Leoncini, along the Portico of the Doge’s Palace, in St. Mark’s Square and on the pier, it is forbidden to sit outside the spaces specifically designated for this purpose.
It is in any case forbidden to stop to consume food or drink with the exception of the areas in concession to the Public Exercises, or to throw or leave on public land papers, jars, bottles and any type of solid or liquid waste.
Translation:
Other forbidden acts
On the whole territory of the Municipality of Venice in a public area, or an area that is open to the public, it is also forbidden:
- to clean yourself, objects or animals
- to throw paper, liquids, powder or other objects
- to place decorations, swags, lights without a specific permit
- to free or to abandon any kind of animal
- to play any kind of game that can cause harm or annoyance, outside of the places specifically meant for that
- to practice dangerous or annoying sports
- to place containers or objects under public fountains
- to enter the grassy areas, to pick up flowers, to fiddle with the plants or otherwise to damage them
- to step or to sit on the flower beds, in the grassy areas of parks and public gardens, as well as to stop/stay on the grassy areas with any kind of vehicle
- to climb the trees, poles, gates, buildings, monuments, lamps of the public administration
- to lay or to mount on the benches
- to swim or to get wet in all the rios, canals, the San Marco basin, and anyways in any kind of water located near to the city
- to get undressed or to dress up in the streets and in city spaces
- to get undressed or to dress up in the streets and in places that can be seen by the public
In San Marco Square, along the arcades and the stairs of Procuratie Nuove and on their extension towards Ala Napoleonica and Libreria Sansoviana, in Piazzetta dei Leoncini, along the arcades of Palazzo Ducale, in Piazzetta San Marco and on the wharf, it is forbidden to sit outside the spaces specifically meant for that.
It is in any case forbidden to stop/stay for eating or drinking, excluding in areas that have been rented to businesses [translator’s note: this basically means bar, pubs and restaurants, their premises and their tables even if located on public soil], or otherwise to throw or to abandon paper, cans, bottles or any kind of solid or liquid trash on the public soil.
Article 49 septies
Original:
Hindrance to road traffic
Without prejudice to road traffic regulations, on the most popular pedestrian routes in the territory of the Municipality of Venice, as identified by a specific trade union ordinance, it is forbidden to stop without justification, causing a hindrance to traffic, with behaviour such as begging on public land or accessible to the pubblico.
Salva l'applicazione delle norme penali, ai contravventori al presente divieto, qualora l'intralcio sia finalizzato alla raccolta non autorizzata di danaro o altre utilità, si applica la sanzione amministrativa accessoria della confisca del danaro che costituisce prodotto della violazione, come disposto dall'art.20 della legge 24 novembre 1981, n.689, previo sequestro cautelare ai sensi dell'art.13 della citata legge n. 689/81.
Traduzione:
Ostacolo al traffico
Ferme restando le norme sulla circolazione stradale, è vietato fermarsi senza motivo sui percorsi pedonali ad alto flusso turistico del territorio del Comune di Venezia (così come individuati da un apposito provvedimento del Sindaco), causando impedimenti alla circolazione, con comportamenti quali l'accattonaggio su suolo pubblico o accessibile al pubblico.
Ferma restando l'applicazione della legge penale, se l'impedimento è finalizzato alla raccolta non autorizzata di denaro o ad altre utilità, ai trasgressori di tale divieto verrà comminata la sanzione amministrativa consistente nella confisca del denaro ottenuto con la violazione, come previsto dall'articolo. 20 della legge n. 689 (24 novembre 1981), sotto forma di sequestro cautelare ai sensi dell'art. 20 della legge n. 689 (24 novembre 1981), sotto forma di sequestro cautelare ai sensi dell'art. 20 della legge n. 689 (24 novembre 1981). La versione inglese del sito web è scarsamente tradotta in quanto il verbo italiano sostare è una combinazione di “to stand” e “stay”. Con la connotazione di occupare uno spazio_ (significa anche “lasciare l'auto parcheggiata”).
Non significa che non si può stare fermi a guardare gli edifici, né controllare il telefono. Significa che non si può occupare uno spazio per mangiare (come un picnic) o per ostruire il traffico pedonale.