Hi tinatina,
Welcome to A2K!
Can you tell us more about when this happens and how it happens? Like, is it typical baby "spit-up" (gross stinky milk-mixture) or actual spit (clear stuff)? Does it happen at certain times, and if so, when?
At five months old a baby isn't doing anything to be mean -- but she may be enjoying the entertaining reactions!
Lacking other info, that's what I'd start with -- just calmly and quietly clean up and say "no" or something. Don't make a big production out of it.
Then focus on prevention, like keeping her to yourself for a while after she's eaten (and doing the calm, quiet clean-up if she spits up). Get a lot of cloth diapers and drape one over your shoulder when she's in prime spit-up mode. Babies just do spit up -- I didn't wear anything dry-clean-only when my daughter was a baby for just that reason. If you're concerned that the spit-up is excessive, talk to your pediatrician.
Good luck!