Accidentally sent Bitcoin Cash to Bitcoin address held by a third party
You'll have to negotiate with your service provider whether they can help you. 

Whoever holds the private keys for your Bitcoin wallet should be theoretically able to generate a transaction that sends the BCH from the Bitcoin address to wherever you wanted them to go.

In practice this may be difficult or infeasible: To keep large quantities of Bitcoin secure, the private keys are often stored in hardware devices or otherwise hard to access. The service provider will likely at least need to manually intervene to help you. 

If the Bitcoin address was SegWit (which I wouldn't expect in this case), it might be even more difficult to recover, as you'd need the help of a miner, additionally.

It is unfortunate, that the Bitcoin Cash developers used the same address format as Bitcoin, even after this problem was well-known and in the media recently with Litecoin switching to a new unique P2SH address format for this very reason. However, I've read that they are working on a new unique address format, so at least this should stop happening in the future.