Ahaa! The manual shows both 'zEQ' and 'ParaEQ' settings, which are exactly what you need on pages 21 and 22 - I'm assuming you have a hard copy version?

Anyway, the para(graphic) EQ should be fine to notch out the rogue frequency...

Also try a few of the zEQ presets to see if one is better than another - you may find you're using one of the 'vocal mic' presets which probably is totally wrong! There will be a mid-peak for vocals which could be causing your feedback.