Fellow user of Blizzard's unit frames, I use different addons to track my debuffs on my target as well as buffs it has. I looked for a way, or an addon for that matter, to hide these buffs and debuffs on the target and focus frames, but nothing existed (atleast post-2.0.3).
HideBuffs (in its 0.01a version) hides buffs and debuffs of target and focus frames.
(I'm sure the code isn't 100% efficient, especially the UIParent:OnUpdate(), but I will work on that.)