Sometimes when you block some content, a white space will remains.
Sometimes that is because other elements reserved a margin for it or is positioned to leave a white space for it.
Some other times, you can remove the white space as well with element rules, but then it may mess up site layout: out of place navigation bars, content overlapping each other, etc. So we must leave them be.
Most of the time the offending style is easy to locate and easy to fix, if there is such a capability.
I know this is not strictly ad-*blocking*, but we are (I am) seeing these problems more and more as css design becomes ever more popular then before... A company's site even have fixed size frameset that, once the ads are removed, really leaves some big big gaps.
Any chance? I only need to be able to change "display:hidden" to something else, like "margin-right: 0"...
Sorry, I don't think custom CSS rewriting belongs in Adblock Plus (and that's basically what you request, "margin-right:0" will hardly be sufficient in most cases). There is a specialized extension to do this (Stylish), the value for blocking ads is not really high, and dealing with security issues because of that is just not worth it.
I think I asked about this a while back. Wladamir chose to minimize bloat and keep ABP good at what it does best--blocking ads.
A while ago, Comedy Central's navigation was totally messed up if ads were blocked. I posted some fixes on userstyles, but I think website creators have wised up to the fact that some people block ads and design their sites accordingly.
So we must leave them be.
