Is it possible to block employees from deleting screenshots?

Yes, it is possible to stop employees from deleting their screenshots.

The organization owner/admin can turn off the option of deleting screenshots for the users. First, Go to Settings> Activity.

Now, Turn off "Allow users to delete screenshots".

Now, Employees won't be able to delete their screenshots from their side. Only admins/owner will be able to delete the screenshots.

Stay productive!