http://lh6.ggpht.com/_w4P-Pns70xU/TMYeoB3Pt0I/AAAAAAAAATI/QTRuejq5LJI/s640/helmlesscss.png
1. Open up the armor's edit window.
2. Click on the edit button next to "Biped Model List".
3. Delete the helmet from the list by selecting its entry and hitting the "delete" key.