you should just be able to erase the configuration.
found this from the manual...essentially hook a console up to it and press any key when it boots to be presented a low level menu.
To set your switch to the factory defaults, do the following:
Warning
This operation deletes existing switch configurations.
1. Attach a terminal to the RS232 port located on the front panel of
the switch and begin the terminal emulation program.
2. Press <Reset> located on the right side of the switch?s front panel.
3. Immediately press any key when you see Hit any key to run
diagnostics or to reload system software. A menu
then displays.
4. Select D from the menu. The following warning message displays:
WARNING: This will erase all current
configuration data!
Continue? Y/N
5. Select Y.
The system displays: All configuration data has
been reset to factory default values.
Press <B> to boot the switch software
http://www.alliedtelesyn.com/datasheets/8118-24-26xl_ig_c.pdf