How to check whether the ad hoc approver is a particular user by setting approval rules?

We want to set a rule in BPM to check whether an ad hoc approver added by expense submitter is an appropriate user to do the task. Is there any condition we can set in approval rule to restrict this, so the system can auto reject if a wrong user is added?

Also, can we set rule to auto reject the expense if there's no approver/user being added in the approval path?