holding menu + "-" button on your remote for 6 seconds should take you to Factory Restore menu.

Factory restore will delete everything you have on your ATV.

if that fails let me know, there is a way to do it via USB and your recovery dmg , but unless you messed with partitions you should be fine with menu + "-" and a factory restore.

Hope it helps
Zeljko

__________________
Officially Unlocked 3G, Model MB489SO, 2.2 --> 2.2.1 --> 3.0 B4 --> 3.0 B5 --> 3.0 GM
Macbook Pro 15.4' Core 2 duo 2.2GHz, 4GB RAM
aTV 40GB, 2.0.2 (2.3.1 OTB),USB support, rTorrent, nitoTV, XBMC 9.04.1,
OpenSubtitles_OSD for XBMC and Plex
USSD for Mac