Just to sum it up,
I guess I'm looking for a function that get the account name on visit of the record and not on modification... does that exist?
Seems like a reasonable approach but it's not the only option. The user did indeed change the record by clicking the check box, but whether that "change" should update this auto-entered field or not depends on whether or not it creates an issue for your data.
It's not the only way to drop records out of the portal after the assigned task has been dealt with.
Once again thanks for responding. Ok, I'll stick with this for now as it does not really matter if it shows that a user changed something even if they did not really commit any changes!