It seems like this place has been ripping off tourists for some time now. When we were ripped off I couldn't wait to post a review here of our experiences. I was quite surprised to find that some 28 other contributors had already posted: all with the same or similar experience.
Reviews are here:
The cafe doesn't seem to have a website, so there was no way I could tell them exactly what I thought of them. Now I know that the ripping off of tourists has sadly become something of a national pastime in Prague, but these guys have become exceptionally good at it.
I found a site that claims to be the official travel and tourism guide for Prague, and they had a "contact us" page.
I've used it to lodge a complaint about this cafe, and pointed them to the reviews here on Trip Advisor. I don't know if it will do any good, but when there's a possibility that a local authority may take an interest in them, then it's worth the effort.
If anyone else here has had similar experiences with Cafe Boulevade, I'd appreciate it if you also write a comment. You'd be helping the cause and possibly benefiting other TA members contemplating a trip to Prague.