The difference between ban and suspend isn’t a temporal difference. Here’s the Cambridge dictionary definition of “suspend”:

to stop something from being active, either temporarily or permanently (see: dictionary.cambridge.org/dictionary/…/suspend)

Here’s the definition for “ban”:

to forbid (= refuse to allow) something, especially officially (see dictionary.cambridge.org/dictionary/english/ban?q…)

The difference between the two is the subject: an active process or service can be suspended, but something specific (e.g. an action, object or person) can be banned. Ban also implies a more official act in order to punish someone or prevent something (Johnny was banned from entering the bus), whereas a suspension doesn’t necessarily have that ‘negative’ context (e.g. the bus service was suspended, which doesn’t imply this happened because the bus driver was drunk or something).

In a more Lemmy-specific context, you could say you suspended someone’s access to the platform, or that you banned them from the platform. Neither way of saying it implies anything about the duration. You can’t however really say you suspended someone from the platform, that doesn’t really work.

In this context, I think the direct implication that a ban is handed out because someone did something bad is a lot clearer than when you use the word suspension. Because of that I believe ban to be the more context-appropriate word here. Suspend does not carry that connotation as something can be suspended for a whole host of reasons, none of which have to be related to rule-breaking. For example, federation with another instance could be suspended temporarily until the other instance does (or doesn’t do) something that is required for technical reasons.