When adding the AJAX Marking Block to a course the following query begins to run and then locks MySQL:

When the process is killed it restarts itself. To replicate the issue we do the following:

1] Create a course.
2] Assign yourself the role of Teacher (default) in the course.
3] Add the AJAX Marking Block
4] Check MySQL process list for the query.
5] Attempt to kill this query.

The only way to remove the block from the course is to hit the remove icon in conjunction with killing the process in MySQL.