Below you will find two screenshots of a typical website that uses banner ads.
The first screenshot shows the page after using this script to erase the ad.
The second screenshot shows the page as it normally appears without using this script.
Runs on Firefox and on Google Chrome browsers as of Nov 2017.
After installing this script clicking on any advert/image etc., whilst holding down the "Ctrl" key, erases it by drawing curtains over it. Upon revisiting the page at another time, the advert/image etc. will still be erased. A double Click on an erased (curtained) space temporarily reveals that which was erased.
The script also adds to the GM menu a command that gives access to the scripts options. One of which allows one to set the 'curtain' image to a http image address or a base64 string. It also allows manual alteration of the selectors used to find elements for erasure. Another option allows for complete erasure of items leaving more space for other items on the page (for this, untick the "preserve layout" tickbox).
See this option menu under the browser's Tools menu --> GreaseMonkey-->UserScriptCommands...-->EraseWebElements.
This menu must be accessed on the Google Chrome browser without GM menu, so, to access this script's menu command: ctrl-click anywhere, you should see a small popup, then click on link named Erase Web Elements that appears in the small popup.
Also, to install on Google Chrome Browser, save this script (use right button menu button "Save Link As ..." over the "install" button on this page. Then drag and drop the downloaded script into chrome's "extensions" page which is accessible from the Chrome menu "More tools->Extensions."
WebEraser, Nuke Anything on the web with Weberaser.