Yes, you can.
Edit the /etc/apt/sources.list and enter "stable" instead of "testing" wherever it says just that. It depends on which mirrors you are using however, but trying wont hurt you. Otherwise, go to www.debian.org
and get some new mirrors, with the stable packages.
The problem with going from testing to stable, is that the testing packages has a mroe recent version number, and might create broken packages, since some applications still require the testing packages, some dont. There is no foolproof fix to that as far as I know however. Good Luck.