Surely someone, in the Blades sections, must have seen the throwing of coins from the Blades sections. That being the case, have a word or if you don't want the aggro, fully understand, pinpoint the seat nos., call a steward or report back to United with other, if possible, witnesses and let them deal with it.
The consequences of coin throwing can be life changing, NOT acceptable.
Remember charity collections back in the day at the Lane, Rag Day, St Johns etc., tarpaulin to catch crowd thrown coins. Stood at the front as a kid, painful but at least you knew it was coming and took evasive action. Threepenny bits aren't good on the back of yer nut, experience with a small profit as there were often coins on the terracing as crowd cleared at 1640 hrs.