Look in the ApplicationConversionServiceFactoryBean.java (or .aj). There will be a method along the lines of
public Converter<Exam, String> getExamToStringConverter()
or
public Converter<ExamPK, String> getExamToStringConverter()
Change that so it only includes the fields you want to see.
Alternatively, you can just add a bean style method in Exam.java that returns the string you want and call it as a property. This assumes you've created a method named getMyExamId() that returns the string you want. Notice you'll need to change z to 'user-managed'
<table:column id="c_com_mypackage_examino_domain_ExamModule_examId" property="myExamId" z="user-managed="/>