Can iTunes ignore store credit and use a credit card instead?
According to Apple's page, it uses the following order for song purchases:


Song credits.
Gift certificate, prepaid card, or allowance credits.
Credit cards.


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:


Gifts will deduct money from the credit card on your account; they will not use up song credits nor gift card credit. This means that you do not need to open another account, just add it as a credit card and gift it to yourself.
You cannot send a gift to the same email address as your iTunes account. This can easily be circumvented by sending it to one of your other email accounts.
The easiest method of gifting to yourself is to select the print option. It will show you a code which you then use to redeem the song. It will allow you to redeem the code on the same account.