You'll have to do a factory reset, which will also remove all downloaded apps and configuration data. To do so, boot into recovery mode and then follow the prompts. Research online if you don't know how to access Recovery, as it varies among different devices. For example, on my Samsung Infuse it was Press & Hold vol-up, vol-dn, and pwr simultaneously; on my Galaxy S4 it is vol-up, home, and pwr simultaneously.