Who heard this accusation? As for it to be slander then surely the person who was privy to that conversation other than the bloke who signed the letter who have to be named and be witness to the supposed slander?
For it to be libel, then it would have had to be printed in the public domain, which it wasn't - aside from by him.....
As for the supposed allegation, it's nothing of the sort. It's a statement of fact. The tweets, the account, the e-mail address, the home address are all his, therefore the club have decided that's enough information with which to decide to ban him from their ground. They've not accused him of racism, they've just listed the exact tweets and said that they contravene their terms and conditions.
It's akin to the owner of this site or a mod banning you for making racist posts, you could deny it was you and that someone hacked your account, but they'd still ban it and you'd have no redress. Only if they then posted on their site that you were banned for being a racist could you have grounds for complaint, Until that point it's a private matter.