You can use the method Scribe mentioned, by stopping GPU-crunching every time you uses the computer.

Another method is to use <exclusive_gpu_app> on your games. If you uses the computer frequently for other things than games and the GPU-crunching doesn't interfere too much with these other uses, only suspending on the games is an advantage. The disadvantage is you'll need to know the name of the executable for all the games, and you must manually add these to cc_config.xml

cc_config.xml is located in your BOINC data-directory, if it's not present you'll need to make the file. Use Notepad to edit the file. Here's an example-file stopping GPU-crunching on Half-Life 2 & Portal 2:

<cc_config>
    <options>
        <exclusive_gpu_app>portal2.exe</exclusive_gpu_app>
        <exclusive_gpu_app>hl2.exe</exclusive_gpu_app>
    </options>
</cc_config>

Just add more lines with <exclusive_gpu_app> to suspend on other games.

Oh, and in case you've got a game or other program there cpu-usage also interfere with usage, use <exclusive_app> on these.