In terms of malicious insiders committing fraud, can anything "really" be done?
There is a popular quote from the 2003 version of the film The Italian Job that comes to mind when I read this question. "I trust everyone. It's the devil inside them I don't trust." The threat from insiders, being fraudsters or otherwise, has always been there - certainly before security companies started talking about them. What has changed, and this is in direct response to the question, is that - yes, there are things that can be done, many in fact.
First, let's look at the advantages insiders have: Trust and access. Consider a college intern, Sam, working for a large financial organization. Sam's job requires him to enter payees into the corporate database, and to do this job he his granted access to the database. Sam eventually realizes that his access doesn't just allow him to create payees, but also pay those payees because of flawed access controls, lack of segregation of duties, poor policies, etc. Sam begins generating false payees associated with a PO Box he set up, and then has checks sent to that PO Box.
This is pretty low tech - not likely to make its way into the newest James Bond film. But Sam's activities are also representative of how many fraudulent, insider incidents are perpetrated using activity that stays below the radar of most security controls. What is needed to address this are solutions designed to do just that - detect suspicious activity carefully concealed as legitimate, normal, and otherwise boring.
Detecting fraudulent, insider activity requires a combination of network and data security. While firewalls, routers, VPNs, and IPS solutions provide tremendous value they are just the peanut butter. Similarly, while applications, database activity monitoring (DAM) solutions, and identity solutions provide valuable insight, they are only the chocolate. However, by putting network and data security solutions together under one monitoring umbrella with SIEM (Security Information and Event Management) you have a delicious treat, and a comprehensive approach to mitigating fraudulent, insider activity.