Hi Alessandro,
quoting upstream [1]:
Suspend processor and network usage when these applications are running: When programs in this list are detected in memory, BOINC will stop working on them. A separate option to stop working on the GPU only, but allow work on the CPU to continue, will come in a future version. Until that time you will have to hand edit the cc_config.xml file and use the <exclusive_gpu_app/> flag for that.
[1] http://boinc.berkeley.edu/wiki/Local_preferences#Exclusive_applications
So I guess we just need to wait!
cheers,
G.