Is it mutar for a man to pluck his beard hair with his hands or does this fall under the issur of destroying the corners of the face?
Many Orthodox Jews, including Hasidism, refrain from cutting their beards altogether, and, with the exception of occasionally trimming their mustaches when they interfere with eating, never cut their facial hair. Those Orthodox Jews who do shave their facial hair must utilize electric shavers as opposed to razors.
To your question if plucking falls under the Issur of destroying the corners of the face, as long it is not done with a razor, Issur does not apply. The Mishnah interprets this as a prohibition on using a razor on the beard. See Eidut L'Yisrael P. 145
You may want to check out this post if removing hair from a man's body falls under the Issur of "Lo Tilbash".
The credit here goes completely to Danno who linked this psak, but for the sake of answering the question here: http://dinonline.org/2015/05/15/plucking-facial-hair/
Plucking facial hair in a way commonly done by men as well as women is permitted. It is forbidden for a man to pluck out white hair from a colored beard [Y:D 182:6]. Plucking hair from the sideburns should be avoided as according to some opinions this is a problem of הקפת הראש, removing sideburns. See Gilyon Hashas Shavuos 2b, and Shu”t Chasam Sofer Y:D 139.