If you are in the United States, I believe it may be illegal for your employer to garnish your wages without a court order.
Even if the related provision in your contract is valid, the contract itself says they can't dock your wages until 30 days after your termination. 30 days after termination, you won't have any outstanding wages for them to dock! It's a stupidly written contract, and the stupidity is in your favor.
If you are desperate for money right now, feel free to call their bluff and threaten a lawsuit. Get loud enough about the breach of contract and they are very likely back down and give you the final paycheck.
Afterwards, of course, do make sure to pay what you owe!