Q: How can I delete an iTunes account from my iPhone so it's no longer associated with my device?

If you mean that it has the 90 day association : http://support.apple.com/kb/HT4627

then you can either wait for the 90 days to complete, or you can try contacting iTunes Support and see if they can remove the association : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page