There are times when I've got some store credit from a gift card/certificate on iTunes, but I want to buy something without that.
I want to buy something with the credit card, regardless of what's prepaid on the account.
Is this possible?
According to Apple's page, it uses the following order for song purchases:
Thus, it would appear that you cannot charge a credit card without using up your credits or gift certificates first.
However, similar to what @Nick said, you could just open up another account, add the credit card to it, and then gift the items you want to your original account.
Update: Based on my own tests of self-gifting, these are my findings:
I think if you "gift" something you will use a credit card instead of store credit since you can't buy a gift with store credit.