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It's possible that the banned user is interested in a service someone has posted about it and wants to use it.
As the banned user needs to know if the announcement poster is trustworthy enough, he/she should be able to modify his/her trust list.
That's only a guess from you. The problem for us here we really don't know which features a user can or can not use and its reasons when they are banned. There are no clear direction from system documentation. Or even if it does then I do not think it has been updated. Theymos has made many changes to original SMF and I can confidently say that he did not bother to document anything at all 🤣