man dpkg reveal that you use the option "-r" to remove a package.
So why not try :

dpkg -r opera-static_5.0-0beta7-20010426-023_i386.deb

But I'm no expert at dpkg: I prefer to use apt-get.

If you install it that way you can use Synaptic too to un install the program.

Use dpkg -r opera-static_5.0-0beta7-20010426-023_i386.deb
to uninstall Opera.
The -r stands for remove, as the -i stands for remove

Try apt-get remove opera-static_ 5.0-0beta7- 20010426- 023_i386
Without the .deb