Due to the lack of a "CPU only" switch in the Boinc-Settings I made a small cc_config.xml which should make BOINC never use the GPU at all, even if GPU only tasks from projects where I can't configure CPU-only are incoming:

<cc_config>
<options>
<exclusive_gpu_app>explorer.exe<exclusive_gpu_app>
<exclusive_gpu_app>svchost.exe</exclusive_gpu_app>
</options>
</cc_config>

But after restarting BOINC I get messages that the syntax isn't right, BOINC said that <exclusive_gpu_app> and both of the closes (</options> &</cc_config>) can't be recognized...
But the syntax should be right after viewing https://boinc.berkeley.edu/wiki/Client_configuration#Configuration_file

There's a missing / for the closing part of the first exclusion.