The bigger a community gets, the more disruptive members you will have.
That's the law of averages. Some people will be disruptive. You can remove this behaviour, but not control it.
Three things. First, removing content doesn't change the people. If you want to change the behaviour of members, use subtle influences.
Highlight the number of bad posts that have been removed, time since a bad post, number of people helping remove the bad posts. Highlight the behaviour you want members to emulate.
Second, if you're going to make a big announcement, make it big and bold. Nobody reads communiy announcements. It appears in inbox and gets deleted immediately.
I prefer Wikipedia's blackout approach. No forums for 2 weeks. Just host a single message with a date for the relaunch.
Third, the challenge isn't preventing this behaviour from happening – but removing it before it becomes widely noticed. That means technological or human filters. You need a system where trained members can highlight material that must be removed