USBGuard disabled all peripherals

[123J1232]

I recently acquired a 2011 iMac with Linux Mint installed, the apple mouse was not recognised so i plugged in a working usb mouse which also was not recognised. I looked up potential fixes and installed USBGuard which promptly locked all USB devices out and I am now unable to use a USB mouse or keyboard. I have downloaded Linux Mint Xfce Edition and made a bootable usb stick with Etcher but when inserted into the iMac it is not recognised when i reboot it and with no keyboard/mouse input i cannot get to the boot menu. The iMac just ignores all input and boots to the previous Linux Mint log in screen that was installed when I received it. I do not know Linux so please give detailed instructions on how to proceed. TiA