A bug in upgraded Ubuntu 10.10 (Maverick Meerkat) installations has caused the boinc manager to lose its menubar. As a result the option to control the execution policy is gone.
The same control is available through the commandline,
boinccmd --set_run_mode always
for starting and
boinccmd --set_run_mode never
for stopping.
Aslak Johansen's Weblog 