The client rules can only be cleared by deleting them locally on the clients, and updating their node properties. In the Firewall options policy, uncheck the "Retain client rules" option. When the policy gets enforced on systems, it will clear away any local rules and enforce only what's in the ePO policy.
Another item worth while mentioning is if you have the "Enable adaptive mode (rules are learned automatically)" setting selected, you will just recreate the client side rules again. Then when the policy enforcement takes place the client side rules would be removed only to be re-learned again.
I hope this helps.