i don't know about use in the casino--but don't worry about using up credit--we always managed to spend ours just on the photographs of the family (couldn't send the kids thru the shredder!) as i recall, it was just applied against the balance. HAL could probably address the specifics--call them.........
I can't imagine not be able to use all your credit. Between drinks, wine, spa, shore excursions, etc., it should be gone in no time. From what I understand from my TA, it can't be used in the casino. It was my first question when I found out we had a credit!!!!
Shipboard credits show up on you "onboard account", and is deducted from any balances for anything charged to that account.
So.. if you were to "purchase" chips or token in the casino using that account, the credit would apply towards. Though on Holland America there is a 3% service charge for doing this, so it does have a cost.
Regardless of that, you're likely to spend enough on pool-side drinks, wine with dinner, photos, bingos, excursions, or in the onboard shops, to eat up most shipboard credits.. I'd think.