Ruined my FP Guard faction... sigh.

[Sorn]

If you did it by hunting high keep guards, I am thinking you also tanked your faction with the corrupt guards over in Qeynos as well. That's also perfectly easy to get around, save for if you dare to wander into the territory of Lieutenant Dagarok, who will HT you for a great deal and then proceed to beat you to a pulp.

If you want to raise FP militia faction without ruining everything else, try these quests:
(must be at least dubious to FP rogues) https://wiki.project1999.com/Note_for_Rebby
(must be at least dubious to FP rogues) https://wiki.project1999.com/Note_for_Janam
(indifferent to HPH guards after killing lots of orcs/gnolls in the zone) https://wiki.project1999.com/Orc_Scalp_Collecting and https://wiki.project1999.com/Gnoll_Scalp_Collecting
(must be in good standing with Erudin) https://wiki.project1999.com/Inert_Potion_Quest

If you happen to foolishly end up KoS to other factions, this guide I never quite finished but still works for lots of things may help: https://wiki.project1999.com/An_Iksa...Through_Murder