If you want to completely disable logging, you can disable logging in the root log.
<root>
<level value="OFF" />
</root>
If you want to disable Active Record only, you need to find out the namespace that Active Record uses (or if they did not abide by this convention, the name of the log using the active record).
If we are talking about NHibernate, then the following configuration does the trick:
<logger name="NHibernate" additivity="false">
<level value="OFF"/>
</logger>
<root>
<level value="DEBUG" />
<appender-ref ref="LogFileAppender" />
</root>
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