is there a way to lock an issue so that no one can make further changes?

[Nic Brough -Adaptavist- · Accepted solution]

Move it into a status that has the "jira.issue.editable" property enabled in the workflow (and think about adding the "can't comment" one too).  You can see an example if you look at the "closed" status in the JIRA default workflow (the uneditable one distributed by default)

There's actually quite a few of these properties, and you can do things like allow certain groups to edit too - see https://confluence.atlassian.com/jira/workflow-properties-189949.html for the outline