As long as you are signed in as a member, your available store credit/ reward credit will automatically be applied to your next eligible purchase, whether online or in-stores.
Unfortunately, you will not be able to opt out of using your available credit for an eligible purchase. The only way around this is to make the purchase anonymously in-stores. You can do so by telling the cashier you do not wish to sign in for the purchase. However, the transaction will not be recorded in your account and your purchase will not count towards accumulation of spending.