If you're not using any GPU applications, as you describe above, you can always do this: In BOINC, go Activity>Suspend GPU. You'll still get PPS Sieve tasks downloaded by default, but they won't run unless you change your preferences back to "Use GPU always".
Same result, anyway, so if you've got it sorted then I don't see a reason to mess about with it. It can be a handy tool when you want to run GPU taks on one PC but not other(s) though (or, indeed, if you want PRPNet to use the GPU on a machine but not BOINC, which you want to still run CPU WUs); it's a bit less hassle than setting BOINC to use GPUs based on preferences, and then configuring your different machines to be "work", "home", etc. You just select to suspent the GPU (again, only within BOINC) on whichever machines you want, but your other machines won't be affected at all and will simply act on your preferences.