Complex sentence: When the black card is issued, the offending fencer is ejected from the event (or tournament) in which he or she is participating and forfeits all their bouts, even those that the fencer has already won.
Split into simpler sentences: When the black card is issued, the offending fencer is expelled from the event or tournament in which he or she is participating. The fencer forfeits all bouts, even those that the fencer has already won.