Block VPNs for signing legal agreements

Is there a way to have anyone signing the legal agreement behind a VPN block or trigger a notification that it could be fraud? 

We had 2 bad guests recently and the IP addresses on the legal agreements were both blocked by a VPN. I think this could help be a warning of trouble to come. 

Not really - there are many perfectly legitimate people who use VPNs for many valid reasons, and, the best VPNs change IP addresses all the time so it is difficult to detect them.

I would advise against this as well.  I use VPNs extensively and have them automatically activate whenever I'm not on my home network.  As former Info Sec, I also strongly advise their use on any public network, like those in airports, hotels, B&Bs, company networks, etc.