Hi James, welcome to the forum!
You could use the Mango/bin/h2-web-console script to launch the H2 web console, then after logging in with the credentials from the H2 section of your Mango/overrides/properties/env.properties file (or Mango/classes/env.properties) to login and DELETE FROM dataPoints WHERE id=(SELECT MAX(id) FROM dataPoints); You can find more about the web console here: https://help.infiniteautomation.com/about-the-h2-database
Or you could move your Mango/databases/ directory, then start with a clean database and restore a backup. That should give you a little time to poke around before it shuts down due to too many points.