For individuals we charge a very small amount for a key/security deposit because there aren't usually problems. Groups behave differently though. When Group Mentality sets in, people start acting like idiots because their friends are acting like idiots. There's no individual accountability because everyone feels like an anonymous member of a larger group. If "everyone" is responsible, then no one actually has to take responsibility. That's when stuff gets destroyed.
When groups ask to make a reservation we tell them that there is a 25EUR per person damage deposit, due in cash upon arrival. We tell them that we will keep the full amount if there are any damages. Our hope is that at least one person in the group will want their money back enough to try to keep their friends under control. We have found that this has scared some groups away though. (Hopefully that is a sign that we didn't really want those groups anyway and not just a big stupid loss of money)