what coudl Denny have done to prevent Wickham from runnng out on his debts OR eloping With Lydia? Short of knocking him down or challenging him to a duel I can't really see what he could do?

I think that he might well have known that a man in W's circumstnaces woudl not have any intentions of marrying a penniliess girl like Lydia, since he needed money and always intended to marry money if he could... so he might well have siad that W would not have intended to take L to Gretna....