In the expert view for creating a new game offer for the waiting room, you can use the option "Accept same opponent" to prevent that the same user joins your game offer too often.
 
You can choose from three types of restrictions.
The text in green color gives the abbreviated form shown in the Restrictions column in the waiting room:

• X total times = a player can NOT join your game offer, if the player have at least X running games with you.
  Example: If you currently play one game with user A and you choose "2 total times" as restriction, player A can join your game offer only one more time.
  Abbreviated form: SOT[2]
   
  
• X times (same offer) = a player can NOT join the same game offer for more than X times.
  Example: "2 times (same offer)" means the same player can at most join 2 games for a particular game-offer.
  Abbreviated form: SO[2x]
   
  
• after X days = a player can NOT join the same game offer unless X days have passed.
  Example: "after 3 days" means, that the same player need to wait at least 3 days before he can join the same game-offer again.
  Abbreviated form: SO[>3d]