Centre-back Titus Bramble has been fined the maximum amount possible for bringing the club into disrepute.
The decision has been made after the 30-year-old was charged last month with two counts of sexual assaults and urinating in public.
He was also arrested in September over allegations of sexual assault and being in possession of a Class A drug.
However, the club has confirmed that the drug-related offence has been dropped.
Bramble, who is due to Teesside Magistrates' Court on Thursday, has also been suspended by the Black Cats.