Maybe they can.
He said that it can seal up endlessly, but not that it can only do it endlessly.
And I don't get the "why don't they just beat them up" argument.
Sure...they could, but we don't even know what it's like to be sealed up.
The person could just freeze until they are released or they could end up in somethig like a hell.
Meaning that it's so horrible to be sealed up that it's a very good punishment.
Equally good or better than a beating
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"