you said leaf switches right? If so I assume from what leaf switches I've seen, that the leaf part hangs out a bit from the switch. If you are doing this on a wood panel, I would just run a screw on the underside of the wood that doesn't go all the way through on both sides of the leafs, keeping it from rotating.
Edit: I just looked at a leaf switch I had and noticed it doesn't work like that and the center button would still rotate. Sorry.
You might however, be able to use a little dab of epoxy or something similar to glue a little bump, that the e-clip will catch on and then not turn. This is all based on the leaf switch I have, maybe if you post a pic I can be of more help.