I know that we have Trusted IP ranges/ Remote Site Setting for API calls. My scenario is something like, I have my users all over the world and they access salesforce.com via API. One of the user with a shared User Credential is accessing with wrong token and the User record is getting locked. I'm getting the IP from the related list under User but unable to find who it is. For a temporary solution, I thought I can block this IP. With that I observed that we have the feature in SFDC only to define set of allowed IP ranges but not block an IP or an IP range. I may be wrong :-) Any comments or suggestion please.