Some dimensions in the Marvel universe can only be accessed/exited through vertain points or by accomplishing certain rituals. Or, he could just banish him to a place beyond time, so Buu could not act.
Buu is a magical creature in nature, his mind is set to destroy, I highly doubt his mind could be taken over. DBZers have strong will, and have resisted mind control before.
Strange mentally battles "magical creatures" almost all the time. It's one of the key areas of his job description.
A Buu, with polymorphic traits like every other Buu, got hit with his own Candy Beam and was unable to revert. No Buu has demonstrated resistance to such an attack.
"When did I say he couldn't?"
You seemed to suggest that Buu's energy output was more impressive than channeling fledgling Zom.