You did not specify which edition, but according to the BRB from the 6th edition on P 97, characters move into combat within a unit during their own movement phase, so to answer your question, it depends on who's turn it is. You replace an ordinary trooper as specified in the rules.(I interpret that as NOT being a BSB, Musician, or Champion given the rules on page 95) Once in combat a character can't move.
With all of that I would say that a HE character with the COB, can only move in to use the COB if it is out of combat, and a dwarf character has a ordinary trooper in base contact with him. If he is only touching characters, and/or FC models, then the character with COB has to either wait for an opening on the character or get into combat and declare a challenge.(which he can do that turn as he is now in combat) As far as the Dwarf character, if he comes in contact with a HE character with the COB in close combat, then the dwarf is loosing runic items.
I would have to admit given my experience with WFB, it is a VERY rare occurrence where my dwarf characters were only in base contact with all character type models, so I would say that as long as the HE character is out of combat, he has a good chance of neutralizing a runic item.
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