Wow.. I never experience anything like this before.
There were 3 incidences involving in the span of 3 hours when I was there. From underage drinking, a drunk gentleman buying drinks for said minors and finally, a loud disgruntled man getting violent with other patrons. My gf and I promptly left as we feared that someone was going to get hurt. Never in my life have I experience that.
I can understand that a cafe cannot control a customer's behaviour to a certain extent, HOWEVER... perhaps it's time to re-think the whole "alcohol" in a cafe scheme.
I strongly believe that if an establishment presents itself as a quaint parisienne cafe it should follow through in every detail. Alcohol is certainly not needed in a place like this.