Unfortunately I don't believe this can be done with stock ULX. There are three things you could try:
- Modify the ulx slap command to only allow one target player, but that would affect admins and superadmins as well.
- Make a duplicate of the ulx slap command with a slightly different name, then modify that to only allow one target. Give your moderators access to the duplicate command instead of the regular ulx slap.
- Modify the ulx slap command to make it only slap one target from the input if the calling player is in a specific group. This one will do what you want without a separate command, but the groups will have to be hard-coded in the script, and it's a little more involved.