If you did an Erase All Content and Settings, and then installed a new, or different, Apple ID, it should have setup as a new device. However, given the problems you are having you might just call the store support staff and see if they can straighten out the way the iPhone is attaching to an account. They are at: http://www.apple.com/emea/support/itunes/contact.html
When you use iTunes Match or automatic downloads or redownload an account's past purchases then you are likely to tie a device or computer to that account for 90 days : http://support.apple.com/kb/HT4627
You can either wait until the 90 days have completed, 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
But if you do either of those three things on another account then you are likely to start a 90 day association to that account instead.
Do you have copies of what you want on the phone on your computer's iTunes library so that you can sync it to the phone ?