Using the Built-in Filter
Although Opera has no GUI method to control blocking ad banners, it is possible to block web sites through editing a configuration file, filter.ini. Here are the steps needed to enable this:
Create a blank text file called filter.ini and put it in your profile directory (help » about opera » paths » opera directory).
Edit this file and add the following text:
; filter.ini
; This file is part of the Opera browser.
[prefs]
prioritize excludelist=1
[include]
*
[exclude]
http://ad.*
http://ads.*
http://adserv*
http://imgserv.ad*
http://*banner*
http://*Banner*
http://count*
http://par.ad.*
http://*linkexchange*
http://*.doubleclick.net/*
http://*.fastclick.net/*
http://*.burstnet.com/*
http://*.hitbox.net/*
http://*.ivwbox.de/*
http://*.tradedoubler.*
http://view.atdmt.com/*
http://pop.mircx.com/*
Edit your opera6.ini and add the following line to [Adv User Prefs] section (change the path to where you put filter.ini):
URL Filter File=/path/to/filter.ini
You will need to edit filter.ini and add the URL fragments from ad servers you encounter. You can use * to replace a number of characters and ? to replace a single character.
To make it easier to add new addresses to your filter.ini, you can edit your right-click menu to copy the address of a banner and open filter.ini in a text editor. To do this (read EditingINIFiles for a tutorial on editing your menu), edit your [Image Popup Menu] section and add the following (change the paths to fit your system:
Item, "Add to block list" = Copy image address & Execute program,"C:\WINNT\notepad.exe","C:\Path\To\Your\filter.ini"
note: Versions prior to V7.5PR2 had a bug with javascript that would hang the page.
Using CSS to Block Adverts
An additional mechanism to block adverts is to use a user CSS file that blocks advertising banners. Download the adblocker.css from here∞ (made by DarkElf), then follow the instructions at OperaUserCSS.
Other Options?
For much more comprehensive blocking, with many other advanced features, use a dedicated program like TheProxomitron. No browser can match the power of these dedicated filtering software.