Thanks Bruno - I'd advise you open a case with our Support, we probably need to dive into the logs.
Also note - you could edit the syslog.conf file on the ESX hosts, so that they only send Error (and above) severity. Of course you need to visit every ESX host to do that, but it would filter on the host side (and also reduce network traffic, and Collector load)
But anyway this filtering should work on the Enterprise Manager. Please open a support case, and we will investigate. Thanks!