A strap wrench would be better as mole grips would damage the feedneck
they should look something like this
http://www.poolandspa.com/catalog/images/spp-BOA104.jpg
-what did you say has threaded- the feedneck itself or the threads inside the feedneck on the marker?
I had this problem before & I tried everything, I tried heating the feedneck with a hairdrier & that loosened it a little
then I put some WD40 between the feedneck & halo & I managed to get it off with the strap wrench
PS I also tried molegrips
first and it damaged the anno on the feedneck...but if you aren't worried & it's the feedneck that's threaded, then go ahead