Implement 0377
XEP-0377 is implemented in slix, it should be easy enough to implement it in poezio.
Possible implementation
Add /spam and /abuse commands sending the appropriate payload with optional text. Both being included in 0191 would also block the corresponding user.
/spam (maybe also /abuse) could also include denying subscription (if pending), and closing the tab.
(Thanks Link Mauve for the suggestions).
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