As someone who plays the rat engine a lot, (also played against it a few times) I don't think it needs to be banned because the out-activation just isn't that strong after turn 3. also ratjoy in a non hamelin list will cost you 17 points plus killjoy for a grand total of 29-30 stones of your list for a single cruise missile. If your cards aren't good enough the first turn, (1-2 crows plus at least a few 9+'s to make the fat man hit) then your killjoy bomb fails, or only gets one thing. this doesn't take into account the fact that a decent ranged game can put enough pressure on the rats to slow you down a turn.
also, killjoy is Df 4. you will hit him after he activates, and you will likely kill him if you commit 2-3 pieces to the job, honestly it's a lot like having a 10T player throw izamu at you with sensei: you know it's coming, it will hurt, and you cant kill it first. but you can deal with it if you don't panic.
P.S. I am in full agreement with above posts that banning things is the wrong way to go about fixing the game. ratjoy is near the top end of the power curve but its not unbeatable.
-proxy