I ran into this issue last year. What I did was set a timer on when high draft picks / keepers are allowed to be traded. I made it early on enough in the year where everyone should still be competitive and have a chance at playoffs (i.e. can’t trade any picks in the first 4 rounds after week 7/8).
And the second thing to battle this is to change your payout structure so that more of the $ is awarded throughout the season. I added a H2H weekly winner. So if you win your matchup for the week, you get $10 for example. Then regular season winner gets an amount. If you shift more of the money away from the chip and incentivize people to compete on a week to week basis, it certainly helps.
Other than that, you’ll just have to live with some people stock piling picks to win in the future. Personally, I tend to lean win now cause you never know what’s going to happen 2-3 years down the road in football. It changes too quickly. So just cause someone stockpiles picks in future years, doesn’t actually mean it will amount to anything.