I think there is a 2 part solution to this:

Part 1: Salvage and permanently pause the current rating system
1. If you have the logs, dedupe ratings on the same show from the same IP address. Or even just remove all of the ratings after 5 from the same IP to account for some people on shared connections.
2. Permanently freeze this ratings system.
3. Continue publishing the legacy ratings.

Part 2: Build the new rating system
1. Require a Phish.net account with validated email and mobile number that has been active at least 90 days with some forum participation to cast any votes at all.
2. Share the rating distribution data and make ratings non-anonymous
3. To start, only open ratings for the most recent tour.
4. Each month, open ratings for one previous tour with some addition blog and forum content to help encourage people to re-listen to shows. Work you way back to the beginning plus open ratings on all new shows 24 hours after the show ends.