I too am using Opera Mini 7.1 on my BB Curve with operating system 4.5. It seems to be impossible to cancel or change username and password saved on a web page that is not even bookmarked. The "opera:config" suggestion was very useful because this is the only place on Opera pages where I found this info. I also tried the "fold linklists" suggestion after manually cancelling references to the web page in browsing history. The username and password still persist on the web page despite refreshing and even turning the phone off and on again.
BB obviously provides no clues as to where or how to set the Opera Mini configuration. Any news since November 2013? Thanks!