You can uninstall Caribou (which provides an on-screen keyboard), or (as JoZ3 has suggested) you can keep it installed but prevent it from starting up automatically (i.e., "disable" it). The latter may be preferable in that you can still run the program should you find you have use for it (without reinstalling it).

To uninstall Caribou, you can find the package called caribou in the Software Center and remove it. Make sure to heed any warnings about other packages that would have to be removed along with it...though for caribou, this is unlikely to be a problem. If there are packages that the Software Center indicates will be removed automatically when you remove Caribou, and you're not sure whether or not going through with removing Caribou would cause problems, then please post the list of packages here. In case this puts it in a more convenient form, you should be aware that you can get this information in the Terminal by running apt-get -s remove caribou. (The absence of a leading sudo is intentional.)

To disable Caribou from starting up automatically when you log in, for all users, remove the entry for caribou.desktop in the file /etc/xdg/autostart. Since that file is owned by root, you must edit it as root. One way to do that is to press Alt+F2 and run the command:

gksu gedit /etc/xdg/autostart

(This is from this forum thread, found by JoZ3.)

You might want to back up the file first, before editing it, in case you make a mistake. To do that, open a Terminal window with Ctrl+Alt+T and run the command:

sudo cp /etc/xdg/autostart /etc/xdg/autostart.old