Q: Family sharing. Restricting purchases.

I think that family sharing is not the right thing for you. As far as I know you can't restrict adults from charging your credit card.

I don't want my adult family members charging on my credit card.

As Mike points out, iTunes Family Sharing does not allow the use of "Ask To Buy" for adult members.  However, if a member is not cooperating, you as the Family Organizer can remove them from the group:

http://support.apple.com/en-us/HT201081

Note the section on "The family organizer can remove anyone over 13 from the family group at any time."