I did not test it yet, but it seems it's in the events.lua too:
report(select_by_side(side,
_("%s joined RED in %s."),
_("%s joined BLUE in %s."),
_("%s joined SPECTATORS."))
Simply disable that report. Like so ...
--report(select_by_side(side,
_("%s joined RED in %s."),
_("%s joined BLUE in %s."),
_("%s joined SPECTATORS."))