Hide secondary account in Android Market
You can remove the account that it "defaults" to and try to re-add the account. I think there is some type of bug in the newer version of the market because I have the same issue from time to time. it is not the "default" account, and I have like 8 accounts on the device (all google accounts) but it always wants to use this one. 

I found that it "fixes", but usually only temporarily, if I go in to the Application Settings and clear the data for the market app.