One of the most controversial entities (for lack of a better word) in the Linux world today is Mono. I won't go into a lot of detail here, because I would prefer to avoid yet another shouting match about it - there are plenty of those around, if you care to take part in one. What I will do here is simply present the command necessary to purge Mono and anything that depends on it from the Ubuntu Lucid Lynx distribution.
sudo apt-get purge libmono* libgdiplus cli-common libglitz-glx1 libglitz1
At the user interface / desktop level, what you will actually be removing when you execute this command is tomboy and F-Spot. As it happens, I personally don't like either of those, and I have long since found alternatives to both that I like a lot better.
The same command works on the two Ubuntu/Gnome-based derivatives I have here, Linux Mint 9 and EasyPeasy 1.6, with very slightly different results. Mint does not include F-Spot in the base distribution, so you only lose tomboy, and EasyPeasy doesn't include F-Spot but does include banshee, which also gets removed.
The non-Gnome Ubuntu variants I have checked (Kubuntu and Xubuntu) do not include Mono, so this command is not necessary, and is a no-op if you try it anyway.
If you are interested in why one might want to banish Mono, one place to look would be the Mono-Nono web site.