Preventing abuse is definitely a problem, but there are a few things you could do to mitigate it. Taken together, they might be effective enough to make the overall system useful:

1. Dole out "feedback points" based on hours played. This causes characters to be judicious in choosing when to leave feedback and limits the influence a rogue character can exert.

2. Start all characters with a default number of "feedback points". However, in the output from the "show feedback" staff command, hide comments from characters that lack some threshold of hours played. Once the character reaches the threshold his past comments start to actually count. This prevents someone from rolling up multiple characters just to abusing the feedback system. In order for their feedback to count they have to actually play those characters for some length of time. The hour threshold should not be public knowledge.

3. Ignore mutual feedback between two character when it's the same type. If I enter negative feedback on you and you enter negative feedback on me then both comments are ignored. Same deal when the mutual feedback is positive (in order to nerf quid pro quo arrangements). Ignoring mutual positive feedback will result in some collateral damage (i.e. legitimate comments being ignored) but I could live with that.

4. Let sufficiently high level staff set an "ignore all feedback" flag on characters that they observe committing obvious abuses. This flag would be applied retroactively to feedback that was entered prior to the abuse being observed. Characters so flagged would not notice the flag being set. This is important, because it turns those characters into honeypots. The player keeps trying to abuse the system with that character but all his effort is for naught.

5. Let sufficiently high level staff set a "extra credible" flag on characters who have a record of leaving feedback that meshes with what the staff themselves have already observed. Let's say Zulg sees characters Isildur and Daurwyn acting like total douche bags. He then notices that Humbert has left negative feedback on both Isildur and Daurwyn calling out the same sort of behavior. So he flags the Humbert character as "credible" and all of Humbert's feedback now gets highlighted in the output from the "show feedback" command.

6. This is a no-brainer, but ignore all feedback between characters that have ever connected from IP addresses in the same subnet. This has the happy side effect of rendering it impossible for two characters that have used the same proxy server to leave feedback on each other.