1st Year Keeper League/Trade Question for the #footclan

So I’m the commish in a 12 man ppr 1 keeper league. This is the first year we have done keepers and are going by ADP to determine what round you will lose the player you keep in. And something just got brought to my attention that i hadn’t previously thought about… the #1 overall pick in our league already has Lev Bell. And he brought up him trading back in the first round to say the 12th spot and swapping 1s and 2s with the 12th spot and still keeping Lev Bell at #12 and then picking 13th and then going back to the top of the draft for the rest of the draft. My issue with this is if the guy that has the 12th pick has a top 2nd round player on his team he can just keep his top 2nd round player at the back of the 2nd round and then draft first overall. So basically it would be like both guys get the 1st and 2nd picks of the 1st and 2nd rounds and thats kind of a pre draft collusion in a way and doesn’t fair to the league in my opinion. I’m not one that likes to veto trades. My only thought is to say that pick trading is allowed, but you cannot swap picks in a round in which you keep a player. Has anybody dealt with a situation similar to this and have any advice to give.

Thanks in advance!