I would like to put a time-limit on registered users' ability to edit their posts (about 7 days). This would be so that users would not be able to remove their contributions and spoil threads if they became angry with the forum. Would this be hard to do?

RTFM.

$useredit=86400; Parameter defines number of seconds, after which user WILL NOT BE ABLE to edit his message. By default is 1 day (86400 sec.) If 0 or not set, user will be able to edit his post everytime.

Thanks, Team.

If I want forever allow user to edit...what can I do ?

Persian
From the manual:

If 0 or not set, user will be able to edit his post every time.

So set

$useredit=0;