I think ultimately, you will have to ask someone to leave who will not conform to the environment you want to create. It should be rare, and take many warnings before it happens, though. Of course, the more crowded it is, the easier it is to get away with it, (and the less crowded it is, the less people will understand why you are making such a fuss). What else can you do? Greet people (especially newbies and known offenders), and make sure they are aware of the atmosphere you are trying to have there. Have flyers on the tables saying what the rules/guidelines are. Maybe once an hour (or as needed), giving a reminder to the crowd, in place of or after the cortina. In between tandas, talk to the leaders who are not staying within your guidelines (saying something during the tanda is much more aggressive, and only should be done for something egregious). Basically, anything that will take away the excuse that they didn't know that your milonga was different from the norm in your area. Thus the "offenders" will be people who simply don't care about you or your rules.