Hi,
the sid is PHP's variable to remember session id when cookies are not enabled. JoomSEF automatically appends this variable to SEF url, as it tends to be different for each user visiting your site. It's even a security risk using URLs instead of cookies to pass session ids.
If that's not the case with your URL and sid is used intentionally by some component, I'm afraid there's nothing you can do now.
We'll probably make an option to switch this behaviour off in some next release.
Regards,
David