If the user is locked, Devise is going to redirect you back to the failing page (probably the sign in). To change that, you'd have to create a custom FailureApp changing the redirect url only if the error was related to locking the user.
Anyway, I don't think this is the right usage of the lockable module. It's supposed to provide you with lock abilities if the user tries to access his/her account with some invalid attempts, allowing he/she to unlock it using some strategies (time box, email, whatever).
You're locking the user asking for more credits, right? It seems more a new implementation you should work out, than related to Devise's lockable module.
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At.
Carlos Antonio