It is extremely difficult to prove collusion. Unfortunately, with a free site like this, you do run into it quite often. Flea does not get involved with the problems of individual leagues. The best thing is to join private leagues where you know some of the other owners.
It is easy to look up an owner's history on Flea. If they are in a number of leagues with inactive owners, or in leagues where there are a number of empty spots, avoid them.
Also, do not hesitate to e-mail other Flea users to ask about other users. There are plenty of users here who love Flea and will help to make your Flea experience more enjoyable.