This is especially important considering the following recent change:
To accomplish it, I think it's best to add a column to the reviewer preferences database like inactive_editor_probation_last_start to give an editor a two week warning before removal. Then we can use a daily or weekly cron job to pull up any Level 6 reviewers who haven't edited 30 reviews in the past month but who were not already reminded in the last two weeks (i.e. where inactive_editor_probation_last_start < ( time() - (86400*14) ) ) to either send the warning and set inactive_editor_probation_last_start with the current time or remove the editor and send a notification email that they have been removed from level 6.