Removing a Catkin workspace completely is simple: just rm -rf the workspace directory (not the src, devel or build space, but the root of the workspace).
If you've added a source /path/to/your/catkin_ws/devel/setup.bash to your \$HOME/.bashrc, you'll want to remove that for the workspaces you've deleted as well, otherwise you'll get errors whenever you open a new shell.