1. Remove them via your package manager.
2. Reinstall the OS without them.
Which do you want?
(Both are quite annoying because GNOME and KDE have libraries and applications associated with the DE, meaning that you can either manually remove each part you don't want and risk going into dependency hell, or reinstall OpenSuse without any parts and build them up.)
You may not be aware of this, but most Linux distributions boot into an X Window environment by default. startx is not much use unless you disable the automatic starting of X Window. How to do this varies by distribution.
Actually, as the OP said he wanted to run XFCE, the relevant command would be startxfce4, not startx - no-one wants to live with TWM.