difference between visitor/squatter and occupant

[Jane S.]

my tenants' son/grandson 24 y.o. stays at the house almost every weekend fri nite-sun nite with his 2 boys, 1 and 3 yo.

i just found out about it 6 months into lease when i discovered he's getting mail at the house because "his mailbox where he lives keeps getting broken into". 

tenant decided he's a "visitor" but i don't agree. Should he be on the lease? his place is "too small" to have the kids there with him. This is probably an exaggeration. Plus if he lives elsewhere he cant be on my lease. Plus I dont want him moving in with his kids and snarly exwife.

Tenants want to spend time with the boys. but they are living there  I have no idea where their "dad" sleeps. The house already has 5 permitted adults and a teen. plus baby supplies.

Unfortunately the lease doesn't specify how long or repeatedly relatives can couch surf. My painful learning.

They might leave peacably if I agreed to refund a certain p.c. of the security deposit. ON the other hand they might trash it.

Should I just man up and be happy to non renew their lease in a few months? I resent their using my asset to solve a non paying ****up's problems and making itmy problem.