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).
To upload designs, you'll need to enable LFS and have admin enable hashed storage. More information