I'm just guessing so be aware this is untested
"only guest" might be "guest only"
[share]
path = /share
public = yes
only guest = yes
writable = yes
then in global
make sure there is no:
security = user
and put a line:
guest account = nobody
i'm guessing you already have an account on your machine "nobody"
so just
chown nobody /share
seems like that aught to do it but write back if not