Can I set it so that Boinc NEVER uses my GPU

I unticked
"Use GPU while Computer is in use"
and set
'In use' means mouse/keyboard activity in last" to 9999 minutes (nearly which is nearly a week (6.9 days).

Use these instead, they're located under MilkyWay@home preferences.

Use NVIDIA GPU if present
(enforced by 6.10+ clients)

Use ATI GPU if present
(enforced by 6.10+ clients)

As these are per-project values, you need to make a similar choice for any other GPU-enabled project you participate in.

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I am only doing milkyway@home.