There are no real rules as to this issue.
The Trust System is not regulated by the Mods.

There is nothing you can do about invalid negative trust,
other than request the person who gave it to you, to reconsider.

Chances are if you received negative trust, it falls under one of the following:
1. broke a forum rule,
2. attempting to scam,
3. have scammed,
4. begging,
5. being a troll
6. send pm solicitations,
7. spams with or without sig campaign
8. sold account
9. etc etc etc

There are may reasons why to give negative trust, outside of scamming members.


Edit: added the following

There are no rules/guidelines for giving trust, that I'm aware of, except for:
1. do not trust farm
2. do not use your alts to give your other alts or main account, positive trust.

But both there two above, are also not regulated by Mods.