Multiple accounts
The reason for our "one membership per person" rule is that, in our experience, second accounts are almost always created by somebody trying to evade moderation, mislead other members, or avoid responsibility for their own posts, all of which are detriments to other members.
Name changes
Username changes can cause confusion, leaves quotes of posts mismatched with the original posts, make extra work for the "staff" who do our recordkeeping, and produce other problems, but we don't rule them out; we simply consider requests for them case by case.