Attached is my first contrib submission. M-x slime-undefine-function does not help when you wish to remove a specific method. This contrib adds a new function slime-undefmethod. Invoking M-x slime-undefmethod with the point inside a defmethod definition in your source will find and remove the specific method.
I've posted the code before on usenet, but felt it may be more useful if packaged up with slime. Comments welcome. -- Madhu