User using the simple mode only cannot be granted permission to delete from the recycle bin.

Normally users cannot delete documents from the recycling bin even unless they have administrative permissions on that document. However, starting with build Content Studio  4.0 build 1007 a new global group right has been defined that grants a user the right to permanently delete a document given that they have delete permission on that document. In the group properties dialog this right cannot be set in combination with the "Simple mode only" right.

Import another group into Content Studio where the user is a member and give this group the right to empty the recycling bin.

This problem has been fixed in Content Studio 4.0 build 1010.