Resettle colonists is the first part of a two assignment mini-chain, sort of. The other assignment that you are looking for is "Relocate colonists to federation border", or "Deport colonists to klingon border". It take 5 colonists and rewards 500 dilithium. Look in any sector or cluster that is outside the main fed or klingon sector and check your operations tab under department heads.
My suggestion, if you finished all colonizations either sell colonists on the exchange or use in assignments where you get rid of them for a decent dilithium amount.
If you didn't finish all colonization chains and still want to keep colonists, mail a bunch to yourself or ask some abnormal price on the exchange so you get more space for new ones. Yes, I'm aware it's practically the abuse of the exchange system, but per whatever design choice Cryptic's didn't set any max value for different kind of "items" on the exchange.
Start doing your colonial chains. You will need ~150 colonists to complete all of them. The mission that uses colonists is "Establish First-In Colony in X" where X is the name of the star cluster, e.g. Delta Volanis Cluster, Rolor Nebula etc. Each chain has 7 missions, where the 7th gives you a handcrafted blue doff. They also have an 8th mission in the sequence, Support Colonization Efforts, that gives you refugees on Success and purple handcrafted doffs on Critical Success.