[jim sewell]

John , when you say plug , do you mean spark plug or oil level plug ?
The danger with putting too much torque on spark plugs is that they can break off just below the hex and leave the thread behind .
The s/plug may have a gasket or taper , if it is possible to make a sleeve which will fit over the porcelain and contact the area on the inner part of the hex and strike this with a hammer to force the gasket or the taper to compress a little, it may be enough to allow the s/plug to undo.

Drain plugs with a gasket can be done the same way , giving them a sharp crack with a hammer .
Regards
Jim S.