for the synk problem:
time how long people are on the menu and kick them off if theyre there too long
-----or----
have the clients resynk every 5 mins or so
hacking problem:
check the locks and compare to the last loc and if theres a huge change(eg warping) kick them off
----or---
change the mem addys and have them change every server restart
underneeth ground problem:
like the hacking 1 check the lock and if there z position is lower then he lowest point on the map bump them up or respawn
spawn killing:
make it so the users are given health pickups for the first 5 seconds or so
i dunno if it helps but it was just my thoughts