Sharepoint Extranet and logon audit

Hello,

We have SharePoint 2010 extranet set up with 12k+ users all external user are stored in Data base with Farm based authentication, working fine. 

My question is that how can i check if ABC external user has not log in to the site from the last X number of days. So I can remove him, just thinking to clean up this data base since many users are no more accessing this site.

is there any way to automate this process to check if ABC has not log in to htt://extranet , remove that user or generate a report ?

Any Help ...