kinda stupid question but.. .

There is no need for a ruling, this is just the rules: a combat card is played, resolves, then goes to the ash heap. Any lasting effect must be explicitly stated on the card, if only to specify the exact duration.

Otherwise, when does it stop? Why remember the prevent effect for one round only? Why not the whole combat, the whole turn, forever until used,...?

If you really want a "ruling", you can find [LSJ 20001114]

Simple prevention cards (like Skin of Rock) don't even carry into the
additional strike phase. If you were hit for 1 and your opponent played Blur
for 2 additional strikes and struck for 1 each time, you would have to play
3 Skin of Rocks to prevent it all (1 during the strike phase, 1 during the
first additional strike phase, and another 1 during the second additional
strike phase.)

Ankha wrote:

The relative section in the rulebook is:

6.4.6. Damage Resolution

Damage resolution has two steps: prevent damage and heal damage.
First, the minion taking damage can play damage prevention cards (such as the combat card Skin of Rock) if he is able to do so. These damage prevention cards are played one at a time until all the damage is prevented or until the minion chooses not to play any more.
Any remaining damage (damage that was not prevented) is successfully inflicted. The damage is then healed (if the victim is a vampire) or causes a loss of life points (if the victim is an ally or retainer).

There's nothing in there that also the unused prevention to linger until the end of combat. Once you've resolved the damage resolution, it's over and you move to the next step.

It's really strange that this is not explained in the rulebook. This is not readily apparent to new players.

Thank you everybody for taking the time to explain it. This makes it much clearer for us.