How can I stop my friend from using my Apple ID?

Change your Apple ID password as soon as posssible > Apple ID: Changing your password

He won't be able to download content without the new password nor can he change it.

See: Can I share my Apple ID with someone else here > Frequently asked questions about Apple ID

SBJ03 wrote:

my friend has my Apple ID and no self control. Is there any way i can block his device and stop him from using my password???

thx

After you change the password as suggested by Carolyn, you might want to think about getting a new friend that has self control.

Change friend for one that grabs YOU instead of your Apple ID.