Try starting it up with the shift key depressed (= extensions disabled). Nevermind, that won't work
You need to get at the file:
Hard Drive (Name)>System Folder>Preferences>Multi-User Prefs
... and trash it. It doesn't matter exactly how but that's the gist of it.
You could use a startup disk to boot the Mac, (including a utility CD like Norton or DiskWarrior) or else mount the drive somehow and delete the file from another Mac.
Keep in mind that this Mac originally shipped with System 7.5.3. Any OS disk (CD or floppy) from that version up will start it up. You might even be able to create a startup disk by downloading the software from Apple and burning it to CD. You can get 7.5.3 free from Apple here.
You might have to try a few tricks to get it to boot (it's been a while since I've done this), but I would start with putting all the disk images on a CD and trying to boot with that.
UPDATE: Try this first:
Download the System 7.5 Network Access Disk
, uncompress it, and burn it to CD. Then start up with the "C" key down on the Performa. Should work.