Will Hill are a top notch outfit with the backing of a major PLC and you can be 100% certain that the denial of winnings is not due to the fact someone is ahead overall.
They have obviously detected what they think is a 'fraudulent attack' on the casino and as the alleged perpetrator has already withdrawn more than his original deposit are refusing to pay him any more funds. If this 'fraudulent attack' is no more than using the bonuses to their maximum advantage within their T&Cs then it would be a serious blot on this casinos reputation.
Mitch, I am certainly aware that WH is a quality casino (excluding customer service ). That is why i seemed amazed they would do what poster is accusing them of with no explanation other than what they gave.
If they think he is staging a "fradulent attack" they should explain exactly what that is.
PS- if OP is lying he is a scum but my first instinct is always to believe the player.