XF 1.5 Bulk remove custom user titles

I recently did an import to XF 1.5 that is running pretty well. One issue that I've run across is that during the import process, users who had a title assigned by post count ladder had that title converted to a custom user title in XF. As I have around 1400 users I'd rather just bulk remove them than try to go through one-by-one, so that they'll be back to having them assigned by ladder.

I tried going through Users - Batch Update Users to achieve this, but although custom user title is a field there, leaving it blank apparently carries out no action, rather than removing them all. Is there a different way to achieve the goal of bulk removing them all?
 
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