From The Jargon File (version 4.4.7, 29 Dec 2003) :
flamer
n.
[common] One who habitually flames. Said esp. of obnoxious Usenet
personalities.
From The Free On-line Dictionary of Computing (30 December 2018) :
flamer
pain in the net
(Or "pain in the net") One who habitually
flames. Said especially of obnoxious Usenet
personalities.
[{Jargon File]
(1996-08-26)