Coax connection

My directv was installed but a technician.  I now need to unplug one of the input coax cables from the splitter he installed it in.   I need to do this because i'm installing a new junction box to fit all the cables including new hdmi cable.  Unfortunately, I cannot even unplug the coax from the splitter.  There is a small black rubber piece covering the end of it.  Am I supposed to remove it?  I've tried pliers and cannot get the thing to budge.  Didn't know if there was a trick.  Any ideas?  This seems so elementary but very frustrating!