The Advertisement Blocking plug-in gives you the option to either replace banner ads with text [AD] or with transparent image which size exactly matches the banner ad size.
Select Options
Select Plug-Ins
Highlight Advertisement blocking
Click Settings
Click Miscellaneous
Select how to replace banner ads
Click OK.
However replacing banner ads with transparent image greatly increases page display by completely removing annoying graphics, you may sometimes prefer to replace banners with text links because this way you will still be able to navigate them.
About the Advertisement Blocking Plug-in