Overview

Multiple API's

Security

Ways apps can access your Salesforce data

Passing data via URLs

Be very careful of passing your data from your Salesforce org to third party services via URLs. 

Denying

Turn off the API Enabled permission for users that don't need it.

However, In practice this can be quite limiting because many many apps use the API. 

Block individual apps. See this great post by Cloud Sherpas on monitoring and blocking Apps Managing External Apps with Connected Apps - Cloud Sherpas

Monitoring

As a SF Admin you need to monitor all apps that have a connection to your Org. 

Setup > Manage Apps > Connected Apps OAuth Usage.

Your Name > Personal > Connections shows the apps that each user is connected to. 

But that is not the full story - there may be apps that are connected via token. 

Be aware of any apps that are connected via a token. 

If necessary:

Articles

Articles about the Salesforce API.