I agree with what you wrote but the problem going forward in that situation is that you wrote: "If Sally excludes Sam from her trust list, the impartiality of the situation may be called into question and she may find herself excluded by other forum members"
If that is the case then those members backing Sally could be inclined to exclude all the members that excluded Sally as well as those that left positive counter-feedback, the cycle in that scenario will be continuous.
Well hopefully the forum will not become tribalized so much that anyone who excludes Sam will get excluded themselves by Sam's friends/allies. No one should be "untouchable" in the trust system.
These fucking names... Sam and Sally - same first initials, seriously?
Sorry. In the future, I will use names that start with different letters.