[SDPhantom]

The arguments the new function takes is irrelevant to the taint system. It's merely the fact you're replacing the old one.

Instead of overwriting the function, I would secure hook it.

hooksecurefunc("Target_Spellbar_AdjustPosition", function(self)
    if self == TargetFrameSpellBar then
        self:ClearAllPoints()
        self:SetPoint("CENTER", UIParent, "CENTER", 0, 40)
    end
end)

This should cause the TargetFrameSpellBar to snap back to the position you set whenever the function runs, without causing taint.