there is no "should".
[assuming you simply mean when a particular anarchist feels they have been fucked over, and not some universal/moralistic "wrong"...]
i see no reason why anarchists would be more forgiving that non-anarchists. and in fact, since the anarchists that i choose to associate with do not value (or even give credence to) morality or any kind of "turn the other cheek" bullshit, it may well be that anarchists are less forgiving.
as an example, many people see modern civilized society as horrendously fucked, and most know who the most egregious contributors and perpetuators are (corporate decision makers, government decision/law makers, high tech priests, cops, etc). but non-anarchists tend to forgive them, and simply continue participating in and actively perpetuating the system - perhaps with hopes of changing it. anarchists tend not to forgive the powers that be, and in fact those perpetrators tend to be a major focus of anarchist projects, mistrust and hatred.
since personal responsibility and unmediated, direct action are meaningful principles for this anarchist, when someone fucks me over (in a way i consider significant), they will be dealt with directly. that dealing may be anything from a conversation (which may result in me being ok with them) to physical attack or worse. i'm not sure "forgiveness" even plays into it, even if we end up ok after dealing. i'd have to think a bit on what that word really means to me.