Change the logging level of the hibernate package to FATAL (if you don't need ERROR messages):

<category name="org.hibernate">
 <priority value="FATAL" />

 </category>

or the other way is to change the Threshold of the SMTPAppender to FATAL. But note that ERROR messages from non-hibernate packages too will be ignored in this case.

<appender name="SMTP" class="org.apache.log4j.net.SMTPAppender">
 <errorHandler class="org.jboss.logging.util.OnlyOnceErrorHandler"/>
 <param name="Threshold" value="FATAL"/>
 <param name="To" value="admin@myhost.domain.com"/>
 <param name="From" value="nobody@myhost.domain.com"/>
 <param name="Subject" value="JBoss Sever Errors"/>
 <param name="SMTPHost" value="localhost"/>
 <param name="BufferSize" value="10"/>
 <layout class="org.apache.log4j.PatternLayout">
 <param name="ConversionPattern" value="[%d{ABSOLUTE},%c{1}] %m%n"/>
 </layout>
 </appender>

As Jaikiran said, but I'd instead only filter down the offending category, not all of org.hibernate:

<category name="org.hibernate.util.JDBCExceptionReporter">
 <priority value="FATAL" />
</category>

S,
ALR