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The spam.. It's pretty straightforward gentlemen, advertising doesn't harm as they can find servers on hosted tab pretty easily. It's about the way the advertisement was made, spammed ones are definitely not welcomed but banning a guy who advertised his server in someone's pm is pretty much a time waste. Only spammed ads are to be worried of and should be automatically prevented.

If one guy leaves the server then he just joined it just to waste time and not to stay in it and if he reports the advertiser then you can simply ban the guy if he's irritating others.