You can use the <xsd:any /> element together with the Xml Schema Instance type attribute.
Schema
<?xml version="1.0" encoding="utf-8" ?>
<xsd:schema attributeFormDefault="qualified" elementFormDefault="qualified" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:element name="root">
<xsd:complexType>
<xsd:sequence maxOccurs="unbounded">
<xsd:any processContents="strict" namespace="##local"></xsd:any>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:simpleType name="st">
<xsd:restriction base="xsd:string" />
</xsd:simpleType>
Test Xml instance
<?xml version="1.0" encoding="utf-8"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<!-- valid -->
<one xsi:type="st">value one</one>
<emptyone xsi:type="st"/>
<!-- invalid -->
<two name="myname" xsi:type="st">value two</two>
<!-- invalid -->
<three xsi:type="st">
<four xsi:type="st">value four</four>
</three>
</root>
Conclusion
You cannot enforce a simple type in the xsd schema alone.