Any app you download should uninstall, unless there is something fishy about that app. Try turning off the phone and then turn it back on in "Safe Mode" (instructions on how are below). You should be able to stop the app and then uninstall in from that mode.
Should it for some reason still not work, which would be really odd, then you may need to go back into normal mode, save all your personal files, contacts, etc., off the phone and then do a "Hard Reset" of the phone. This will remove anything and everything on it.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.