There are a few button combinations you can hold while the phone is off/turning on.
1. Safe Mode where none of your settings are loaded but you should be able to uninstall this buggy security app without loosing any data (you may have to disable this security apps device administration access if it has it in Settings>security). hold volume down when the boot logo appears (galaxy s4 logo) until safe mode appears in bottom left corner. to get out of safe mode turn off the phone, if its still in safe mode (you didn't use the restart option did you?) some models use a toggle, so try putting it into safe mode again.
2. The Recovery, a basic menu with factory reset as one of the options, hold volume up+home+power until it appears. use vol up/down to move,power to select, no third party app can get in the way of opening this.
3. If that doesn't work your going to need to flash the firmware, you need to get the right firmware file for your model number (i9000,sph-l720,etc) & the right software version (system update) or you risk really bricking your phone. If your rooted you might want to stay on your current firmware version instead of the newest, then again if your rooted you could go into custom recovery & delete the offending apps apk file.
If you got your phone from a carrier you should be able to get one of their shops to flash the newest firmware for you, ask your local carriers store if they can do it or a location that can.