You should be able to edit any rich text that is imported into UR. If that isn't the case, describe in more detail how you are importing the rich text when it is not editable.
HTML is not currently editable internally, but it can be
edited externally, and it's on the list to support for a future release. A couple things that can help regarding HTML banners: 1) You can turn off importing of images via 'Tools | Options | Import (More) | Download Images'; 2) Proxomitron [1] is a really useful (free) app that sits between your browser and the web and (among other things) can strip most ads from web sites before they get to your browser; 3) You can import just part of a web page into UR (potentially excluding unnecessary images and fluff); just select the text on the web page and drag/copy into UR.
[1]
http://www.proxomitron.info/