We recently had a situation where a customer paid off an existing balance on a line of credit at one branch, and then went to another branch the next day and drew the line down again. The check was returned, NSF, of course. Can we place a hold or "freeze" a line of credit until checks clear on large payments? What are your suggestions for eliminating this type of risk in the future?

You could hold the LoC until the funds are verified and clear. I'd back-date the entry for interest purposes so they get credit as of when they paid you.