Cannot prune (delete) thousands of users
 

I'm doing a clean-up as we have about 40,000 inactive users. I tried using the prune function, but I get the "Invalid Action Specified" error. I managed to delete when it's less than 1,000 users, but not more.

We're running v.3.8.2.
I tried adding these lines to the includes/config.php:
ini_set('memory_limit', -1);
ini_set("max_execution_time", "2880");
I tried in several browsers.

We do have some spam mods installed, but I can't believe its necessary having to remove all this to delete these users. After all it works for < 1000 users..

Any ideas how to get about this problem?