mass deleting "public submission" data

Thanks to Didier and the LocomotiveCMS team for building such a nice CMS!

I have a question:

On a customer website there is a backlog of "Contact messages" (almost 6000).

Right now, the "page level block delete" gets the 10 items per page (those being being displayed), which would mean that there are 600 pages of three clicks on each to delete this old data.

Is there a way to display say 50 (or 100) items per page so that there would be 120 (60) pages needed to delete this old data?

One other option is me trying to delete it through the mongo backend, but this is not available to the customer.