You really are prolific

Are you sure you dont get billed, because almost all service providers have work arounds for the "removing the battery" trick. People used to this on calls some time back, so all the providers, upgraded to "time used" method instead of "time end - time start".
I have never verified with gprs. But I recommend that you call up customer care and innocently ask for your balance to be confirmed saying that you are unable to check it yourself becuase of some made up problem. Sometimes they will not let your balance go below zero and will instead debit your next recharge.