There is of course the new ground regarding electronic media, libel and the distributors or publishers of electronic media that can be libellous.
Your ex's ISP might not scan and vet all email (It would also be illegal) and therefore couldn't be held liable for libel too, (as in this case:
Link) but if they are made aware that someone is using their service to distribute libellous material and they continue to allow it, you could probably indicate that you would be forced to act if any further libellous emails were sent via them.
This would probably cause them to revoke her account or warn her of terms of use violations and might be enough of a nuisance issue for her to cease her emails.
Nowadays, losing your email account is tantamount to losing your mobile phone number or your drivers license.