A 17-y-o friend has contrived to get Blu-Tack, now semi-dried, on the inside of a leather or pseudo-leather belt, trousers for the holding of up with.
I've looked at Bostik's Blu-Tack Web pages, at the UK.d-i-y Wiki, and at Cleanipedia, without success; a Google search for [Blu-Tack removal cleaning] finds too much.
What does the group recommend?