Is it permissible to deal with a website that trades real and digital currencies, knowing that some of its members deal in digital currencies that have been said to be forbidden, even though the person himself only trades in real currencies?

It is not forbidden to enter a market or a website that combines permissible and forbidden transactions. Rather, each transaction is judged according to its nature. Transactions free from Sharia prohibitions are permissible, just as dealing with someone whose wealth is mixed (with unlawful earnings) is disliked (makrooh) and not forbidden (haram), according to the more authoritative opinion.