Well, it will depend on how good their slap back script is. If it's just a quickie script, you can just insert some control codes in the middle of the slap...
/me slaps Ctrl-B Ctrl-B nick
Of course, remove the spaces in front of each Ctrl-B so you don't have a 3 spaces in what's shown to the channel. Because you're using Ctrl-B twice, you're cancelling it, so unless someone copies what you typed, they won't know that it wasn't just /me slaps nick.
If they have a more advanced script, it may use $strip to remove control codes. In this case, you might try:
/me slaps Ctrl-K 01,01 - Ctrl-K nick
Again, remove the spaces. This will insert a "hidden" - between slaps and nick. This could help prevent it, depending on their slap back script.
You can also change the l to a 1 as that looks similar in many fonts and could throw off a slap back script.
Keep in mind, however, that a quality slap back script cannot be fooled very easily as it will take into account all these and more.
Personally, I think slapping people on IRC is dumb.
