There are inhibitors of cocaine breakdown - but you shouldn't be using them. Grapefruit juice is not one of them because cocaine is not metabolised significantly in the liver.
Cocaine is principally broken down by pseudocholinesterase & other serum esterase enzymes. Inhibiting these is probably a Bad Idea because they perform functions similar to acetylcholinesterase & hence blockade may lead to unfortunate side effects like muscular tics etc.