Whenever Joven's Ferrets attacks, it gets +0/+2 until end of turn. At end of combat, tap all creatures that blocked Joven's Ferrets this turn. They don't untap during their controller's next untap step.
The second ability will trigger only if Joven's Ferrets is still on the battlefield at end of combat. If it's been dealt lethal combat damage and destroyed during that combat (or has left the battlefield during that combat by any other means), its ability won't trigger and the creatures that blocked it won't be affected.
If a creature affected by the second ability is untapped at the time its controller's next untap step begins, the "doesn't untap" effect doesn't do anything. It won't apply at some later time when that creature is tapped.
The second ability doesn't track the creatures' controllers. If an affected creature changes controllers before its old controller's next untap step, Joven's Ferrets will prevent it from being untapped during its new controller's next untap step.