Demigod can be affected by spells. But here's the thing:

If they are trying to resist when the wizard or witch casts it, then it won't work.

So for example:

If Percy gots hit by Mrs. Longbottom's silencing charm, he wouldn't be able to talk or make any noise with his mouth. But if he were trying to resist it, nothing would've happened.

So when Percy failed to talk, he wasn't actually trying to resist the spell, he was just naturally trying to talk. But when he focused on defeating the spell, it was only too easy to break through it. It was a diamond with no other diamonds to break it with. Then with some effort it transformed into water, easy to pass through its surface.

That's a terrible metaphor/simile thing....

IMPORTANT!!!!! READ THIS OR YOU WON'T GET THIS STORY!!!!!

If a demigod is hit with a Killing Curse, and they didn't resist at the time it was cast, they still won't die. They will be momentarily Stunned and the Mist will make it look like they're dead.