Depends if you mean user or PC
You can't apply policies against users but it's easy against the PC.
1. Create your new policy
2. Find your target PC in the system tree.
3. From the Actions button Action -> Agent -> Modify Policies on a Single System
4. On the end column click 'Edit Assignment'
5. Select the 'Break Inheritance and assign the policy and settings below' radio button
6. Select your new policy from the drop down below.