It all depends on how the other instance responds.

• If their admin don’t want to do anything about users causing problems in other instances then I’d say defederate temporarily (1 month or more) each time their users break our rules.
• If their admin actively encourages this kind of behaviour then defederate permanently.
• If they have rules against bad behaviour on other instances and non-negligible consequences for violations then try to work with them, but still defederate temporarily (~1 week) when our/their moderation team can’t keep up.