Deleting users from AD results in warnings when modifying project roles

When editing the user list of a project role (within any project) loads of log lines like

2013-11-04 13:06:55,329 http-bio-8080-exec-40 WARN local.admin  786x622896x2 7vsz7i 10.10.xx.xx,10.xx.xx.xx /rest/api/2/project/PROJECT/role/10142 [jira.security.roles.OfBizProjectRoleAndActorStore] Unable to create a project role actor. User 'user@mail.com' does not exist.

all these users where AD users in the past but are now deleted. How can we get rid of this? The deleted users are no longer visible in the lists.

My idea would be to have some job running in the background, that gets all user names from AD and cycles through all "role lists" to remove deleted users periodically.

And yes I read https://confluence.atlassian.com/display/JIRA/User+Management+Limitations+and+Recommendations and I cannot avoid that AD users are deleted - we have over 800 users!