Take a look at routing and filtering: http://www.splunk.com/base/Documentation/latest/admin/Routeandfilterdata
You will need a transforms.conf stanza to define what events to ignore and a props.conf stanza to define which sourcetype to apply it to.
You can ignore data by routing it to the nullQueue. In transforms.conf:
[routeInfoToNull] REGEX=(?m)^Type=Information DEST_KEY=queue FORMAT=nullQueue
This will set up everything that comes in to match that REGEX (in this case Type=Information on a newline) to go to the nullQueue (basically /dev/null).
Now apply this transforms to your event logs as such in props.conf:
[WinEventLog:System] TRANSFORMS-SystemInfoToNull = routeInfoToNull
Not that if you're pulling via WMI you will have to apply this to the [wmi] sourcetype. If you want to route data from additional event logs just add more stanzas to props.conf.
An afterthought: There are lots of interesting events logged as informational on Windows that you might want to actually index. Service startup type changes, service start/stop events, Windows update installs come to mind.
It sounds like you want to tune the windows inputs for the forwarding system. See the following link for more guidance: