My iPhone 4S is locked up, how do I get the message to go away?

The message says there's not enough storage and it cannot be backed up. That is not a surprise, I get that message a lot lately. The problem is that I can't get it to close or go to settings. The message box will not disappear when I try to close it, and I can't answer calls, turn it off or anything. I feel like it needs to be turned off and on again. Aside from taking out the battery, which I don't want to do, how can I reset my phone?