Go through all your groups and block he admins. This is a long processes, and it's optional, but I get better results when I do and my posts remain on the groups for days and weeks without being spotted.
WAIT!?!?! Blocking Admins is useless I hear you say. No, it's not. When I move onto Step 4 in a moment I will show you how I post messages and how they are not visible to admins if you've blocked them.