Search-Adminauditlogs - How to excluder service accounts?

Hi Guys, 

We are trying to produce a report showing all the admin activity in the last 24 hours, 

So i Have a simple powershell script running that calls:

Search-Adminauditlog -startdata (get-date.adddays(-1) -enddate (get-date)

Thats fine, but we have some service accounts that run scripts to update mailboxes that i want to exclude from the results.. Any ideas  Ive tried the following..

Search-Adminauditlog -startdata (get-date.adddays(-1) -enddate (get-date) |?{$_.caller -ne SERVICEACCOUNTNAME

But that doesnt seem to work even putting in the DN of the account.. Any way to do this? 

Allan Harding