Do you as a citizen in Prague wonder who tons of drunken youth come to your city just to party, have sex and drink alcohol? Do you believe that your city has other things to offer?
Maybe your city has but it is not how it is marketed. I took the liberty to copy the front page of just one of the travel agencies who offer trips to Prague. If you study the page further you will notice that there is no focus on your culture, your churches. The focus is on partying, having fun and nightclubs.
I know that the police and the hotels had a hard time servicing ten thousands of Danish teenagers who went there last year. In Denmark it is legal and expected that teenagers aged 16 drink alcohol. It is normal and easy to handle in Denmark but we have of course thousand years of experience allowing children drinking alcohol.
I have to say that I was tired of hearing of complains from the authorities in Prague. I have to state that you get the kind of customers you advertise for. Change your marketing strategy and the problems will be over.