Использовать валидатор выражений поля .Пример:
SomeAction.java
private SomeObject object; // with getter & setter
private boolean doNotCheck; // with setter
input.jsp
<s:textfield name="object.field" />
<s:checkbox name="doNotCheck" />
<s:fielderror fieldName="object.field" />
SomeAction.validation.xml
<validators>
<field name="object.field">
<field-validator type="fieldexpression">
<param name="expression">
<![CDATA[ isDoNotCheck() ? true : (object.field != null && !object.field.isEmpty()) ]]>
<!-- OR -->
<!-- isDoNotCheck() ? true : !object.field.isEmpty() -->
</param>
<message>This is a mandatory field</message>
</field-validator>
</field>
</validators>