Because the current method (direct db editing!) is far less than ideal.
If the concern is abusive users (as oppose to well-intentioned but clueless ones), just block them on meta and rely on the OAuth block data?
Change 462534 had a related patch set uploaded (by Framawiki; owner: Framawiki):
[analytics/quarry/web@master] app.py: make it possible to block a user from the site
Change 462534 merged by jenkins-bot:
[analytics/quarry/web@master] app.py: make it possible to block a user from running queries
Mentioned in SAL (#wikimedia-cloud) [2018-09-24T17:53:08Z] <framawiki> deployed 028a292 on quarry-web-01 T205286 T104322