Well in theory create a designer report that will show you (based on the previous x days of interval data that your CMS is configured for) when agent login Id's logged in (i.e. the MAX of row_date). Since CMS only stores data when it received it, if the agent hasn't logged in, it won't be in the list.
Bear in mind that it wont cover agents not measured by CMS and I suggest you validate what you're seeing before you perform any major bulk removals! If you CMS interval data is only available for the last 30 days then login id's not used during that period simply wont appear, so you may need to perform a comparison between this report and the PBX export.
I've just built a report to do this, give me your email address and I'll send yo ua copy
Cheers,
NJ
PS: Please let me know if my advice has been of any use.