reduce session timeout. maybe it will reduce the number of line in the table http://ez.no/doc/ez_publish/technical_manual/4_0/reference/configuration_files/site_ini/session
During this pb did you had a lot of traffic in your web site?
Maybe your have something to check in your Mysql conf.
http://ez.no/developer/articles/tuning_mysql_for_ez_publish/mysql_configuration