Wet capping a leaky shower arm
If the hose fitting can not be removed by you then i would advise you to remove the arm itself.  Get a 6 or 8 inch long 1/2&quot; galvanized pipe nipple
and a 1/2&quot; cap.
Pre-Install the cap on the nipple with two pipe wrenches, use Teflon tape on the treads.
Remove the arm  and thread in the capped nipple into the fitting in the wall, Use Teflon tape on the threads, Tighten with a pipe wrench.
If   the  leak is significant you should turn off you main water supply valve first.