Is it possible to easily remove any desktop in Openbox, not jsut the last one (like from the middle-click menu)?

You may want to check out this link: http://openbox.org/wiki/Help:Configuration

There may be something you can change in rc.xml to implement that feature. If there isn't, you could implement an ugly workaround to get it. Openbox allow binding different programs to keyboard shortcuts, including 'cut', 'grep', 'sed', 'awk', and just about any other command line tool you can use to change your config file on the fly.

I don't recommend doing this, as I have never done anything like this before. I also don't understand why anyone would want to remove only a specific desktop. May you can explain why?

Becasue it seems to me like a much better idea to dynamically add and remove desktops for certain activities that require a group of windows, rather than having a fixed amount of desktops.

Okay, someone else is going to have to answer this one. May you could try a tiling window manager? A few of those have dynamic tagging, which works sort of like virtual desktops. If anyone else has anything to add, please post something!