Given that people often get competitive over games of 1000 Blank White Cards, if anything bad happens to a player, that player is likely to want some points back!
Normally, retaliation is not a problem. Where things start going wrong is when people attack with more force than they were attacked with; the player who sent the original attack may want to send an even larger attack back.
In particularly competitive games, this can go on for a long time; wars can begin over small numbers of points, this is not good, as it may take over the game, eventually leading to...
The MADG situation
MADG is one of the less common play-by-post games on websites. The rules go that every turn, one player removes an item from a long list of items; the last item remaining on the list wins.
This is very similar to playing a game with nothing but You Lose Cards. The is the final stage that the above eventually leads to, although it normally stops long before that.
Fear of extreme retaliation can be very limiting. This is what happens in wars of Unrelated Action Cards and/or Humiliation Cards. A card you might want to use may be blocked by the possibility of someone taking the card at some point and playing it at you, either before or after it's played, or responding with an even worse version.
If someone really wants to take your card, you can't do much to stop them -- but you can certainly slow them down a lot. Here is a way of doing that:
This is a fairly effective method. If there are cards you don't want opponents to use, mark them with a tag in an obvious place—let's say "cake magic"—then make a card that says only you can use cake magic cards.