Rhonas's Stalwart

Once a creature with power 3 or greater has blocked an exerted Rhonas’s Stalwart, changing the power of the blocking creature won’t cause Rhonas’s Stalwart to become unblocked.

If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exert’s effect preventing it from untapping expires without having done anything.

You can’t exert a creature unless an effect allows you to do so. Similar effects that “tap and freeze” a creature (such as that of Decision Paralysis) don’t exert that creature.