"Marked as Sperm": a play on "Marked as Spam", commonly seen in message boards and forums. "Marked as Sperm" is used when the message in question is too odious to be adequately described by the relatively mild "Marked as Spam".
A hypothetical example of "marked as sperm" combined with a slightly edited actual comment:
john_balls1011 @dildoilbravishla: FUCK YOU DICKWAD
(Reply) (Mark as sperm) (Ignore)(Report)