Encountered similar in Istanbul once, Tom.
Descended into a bar on the Taksim Square side of Istanbul, sat down and immediately a flock of waiters plonked down multiple plates of nibbles, bottles of booze and two Russian women sat down either side of me and began to fawn over me. Immediately I stopped the staff opening those bottles, politely removed the girl's hands from me and got up immediately to head for the door as I'd heard about these set ups so many times (also knowing that usually they end up with big time monetary extortion or being beaten up by gorillas at the exit for not paying up 1000s of dollars for a few nibbles and drinks). Cutting a long story short of how I'd ended up at that club and also events afterwards, I got out of that one paying them the equivalent of £7 in Turkish Lira even though they had magically already written a bill within 2 minutes of sitting down for several hundred dollars for things I'd never even ordered. A combination of 50/50 suspicion on my part about the local guys that had befriended me, had led me earlier in the evening put my credit cards and cash in my shoe just in case. If I hadn't, I suspect my account would have been emptied after being frogmarched to an ATM.
Because I was a small and younger looking traveller I suspect they probably did believe that I only had that smalll amount of cash on me. I was very lucky indeed that night. Of course, the two locals were part of the scam and they loiter around the other side of Istanbul luring single male foreigners over to these dodgy clubs.
As for Venice, I went there once but it didn't even come to my mind that there may even be any kind of prozzie scene there.
It seems that the posters above say there isn't anyway.