Officially, https://jira.atlassian.com/browse/CONFSERVER-54928
Unofficially you might want to take a look in network tab of your browser, deactivate a sample user, and work with the HTTP request you see - this way you might be able to automate it with some small local automation script.
Replicating the http forms, and removing unnecessary clutter gets me to
curl 'https://confluencebaseurl.com/admin/users/deactivateuser-confirm.action' \
-H 'Content-Type: application/x-www-form-urlencoded' \
-H 'Cookie: JSESSIONID=55B07A765493F5F108D211F0CDB36CF8' \
-H 'X-Atlassian-Token: no-check' \
-d 'username=testuser&confirm=Disable'
Where:
confluencebaseurl - obviously
JSESSIONID - you need to get this cookie from your current session once you log in, when you log in, you get a JSESSIONID cookie which servers as your authentication, but you could replace this with PAT or basic auth
X-Atlassian-Token - I added this because the data originally contained atl_token, but since this would be automated from outside, we would not have a valid token, so this will skip the token check
testuser - the username
And locally, this seems to be working.
Alternatively you could use groovy (if you have a plugin allowing to), or write a java plugin, but if you just need to do this one time I don't see a big harm in just cracking the http form. Mind you - not official, so do test this first if it does what you expect it to.
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