How do I delete a user

[fkcctv1]

a) How do I delete a user

b) How do I delete a room

[fkcctv1]

I typed POST / _synapse / admin / v1 / deactivate / <user_id> in ssh does not work

[hieronymousch]

You have to do this by running SQL (if you want to really delete).
See matrix-org/synapse#1707 for more information.
Check the sysadmin channel for more help: #synapse:matrix.org

[Josue-T]

I typed POST / _synapse / admin / v1 / deactivate / <user_id> in ssh does not work

It look like more a web request which should be done with curl or with a browser. Did you try it with curl or some other tool like that ?