This is the simplest declaration:

<xs:element name="stop"/>

which allows this to be valid:

<stop>
<junk>I am a junk element with low self-esteem.</junk>
</stop>

This is NOT an empty element.

The type is xs:anyType which is mixed and allows any children.

This is an empty element:

<xs:element name="stop">
   <xs:complexType/>
</xs:element>