Does anyone know if its possible to delete multiple keyframes at once?
Better still, delete all keyframes within a given range?
Don't know of any way to do this "quickly"!
However, if you are no longer using whatever effect is using the keyframes, why not simply remove/delete the effect? I'm pretty certain that will remove the keyframes! (You may need to "cut" your file at the beginning and end of any section where you want to remove the effect so as to create an "object where the effect is no longer applicable, without affecting objects around it.)
HTH
Jeff