This is by design.

When you installed BOINC, you selected the "protected" option. This means BOINC runs whether BOINC Manager is running or not.

Choose "Suspend computation" to control whether BOINC runs.

Also, since you obviously like total control over your computer, you can go to Control Panel -> Administrative Tools -> Services and start/stop the BOINC service from there. But I don't recommend doing that normally. Suspending is easier.

RAID Xero,

Client 6.4 will have lets call it the "Games Setting". You only need to add a line to the cc_config.xml and enter the exe name and BOINC will suspend until you exit the game.

Till then,

Through BOINC manager go to the local preferences, processor tab and untick the "while in use..." box. Not sure if it responds to joystick activity, but it does to mouse/keyboard operation.

There is also the activity menu where you can tell to suspend operation. If the "Leave application in memory while suspended" option is not on, the science applications should unload too.

These settings should stop stop crunching while you are gaming and does not need to exit the client or service.

ttyl

You are correct. You can reinstall BOINC in this way, but it is less secure. Your choice.