The hot water handle in the tub won't turn off, it acts as if stripped. What do I do to fix it?

Pop out the center plastic piece, then remove the phillips in the center, should pull off easy since it is stripped. Replace
You could remove the cold handle and use for now to turn off the hot, to avoid harming the shaft. For tonight use the 1 handle for both.

While Stratmandos solution may be OK for emergencies I like a permanent fix. They make all kinds of handles,(see image) that work on stripped faucet stems. I've shown you just a few. They come with a pointy set screw that bites into the brass stem so one size fits al. If you purchase two of them you can replace both handles and have a matched set. Good luck, Tom

Good advices: also it may be only loose screw that attaches handle to the stem. Screw it in, see if it works. Good luck... Milo

Replace IS a permanent solution. The 1 handle working both was for last night or this morning bath/shower.
Both above post are good.