Per our api doc, the delete enterprise user can take in a "notify" and "force" in the url params. 
notify	 Determines if the destination user should receive email notification of the transfer.
Type: boolean
force	 Whether or not the user should be deleted even if this user still own files.
Type: boolean
I'd suggest to use BoxSimpleUserRequestObject instead of BoxDefaultRequestObject. You probably can add a private method: 
private void setForceDelete(boolean force)
and a static method: 
public static BoxSimpleUserRequestObject deleteUserRequestObject();