The blog post Producing readable log4net output has some good ideas for excluding the Exception StackTrace from LoggingEvents.
In my case I used loggingEvent.GetLoggingEventData(FixFlags.All) to get a copy of the LoggingEventData. I then cleared the ExceptionString in the LoggingEventData and used this to create a new LoggingEvent for use in RenderLoggingEvent.
