You can easily disable inheritance, but this affect everyone on the domain.
If you want to BLOCK only a user / group you can do this:
Open Group Policy management mmc, select the OU where the policy is applied, select the policy. On the right side, there are 4 tabs, select the last one (I think is DELEGATION), on the right corner (bottom), click ADVANCED. On the next windows, click ADVANCED again and add a new user (the user or group you want to block), now check the box FULL CONTROL on the column DENY. To be honest, you can simply DENY the penultimate setting "APPLY GROUP POLICY", but deny everything just to be sure.
After that you can use GROUP POLICY RESULTS to create a resultant policy which should show you that a specific policy is blocked. (Access denied).