Create a new Organizational group (lets say Exceptions group) under the exisiting Organizational View.
Add all the machine names that you want to exclude from patching to above created Exception group.
Make sure all your target machines for patching are under the exisiting Organizational view and under some group ( lets say Target group) .
Any machine can only exist either in Target group or in Exception group under same Organizational View.
When you apply patch policies, apply to only taget group and not to entire Organizantional View. This is how you can prevent patching a exception computer.