Turn off red, wavy underlines for spelling errors
1. Select Options on the File menu:
2. On the Proofing tab, click the Hide spelling and grammar errors check box:
3. Click OK.
This just turns the underlines off; it doesn't prevent PowerPoint from checking spelling as you type.
To stop check spelling at all, just click Check spelling as you type check box:
Turning off Check spelling as you type relieves PowerPoint of a small processing burden, making it run a bit faster.