Possible solutions I can think of, though no one shoot me, this is off the top of the dome...
1. Make the 5v5 a slightly higher stake for the season win, thus people wanna get 5 teams and try to get that better prize..
2. Move it down to single Q for 5's with the achievements still on. This would create a variety of teams. Especially, say 30 people Q for it...You've got....(SOMEONE CORRECT MY MATH IF I'M WRONG, I DON'T KNOW IF I DID THIS RIGHT, DISCLAIMER, I FAILED MATH A LOT LOL) about 900 combos. That's assuming one faction though. But the point stands. Then you can't have the ban for win trading because the odds of landing against the EXACT same team, let alone consecutive times, is stupid low. I'm not trying that math. But the point there.
3. Shut it off entirely.
Problems:
1. This seems unfair to the other ladders, but if you raise the stakes of all ladders, people will still go for the easier to form teams.
2. My math is probably wrong. Again, don't shoot me.
3. No one wants that.
That's what I got.