Here is the thing:  there are a lot of ways to detect VPN usage.  IP logging, tracking cookies, device fingerprinting, VPN IP blacklists, you can even detect VPN usage if you know how to inspect the TCP/IP MTU/MSS configuration on packets arriving on a machine.  Just to name a few.
Here is the other thing: all of the detection techniques have documented countermeasures that can be used to bypass detection.
Are VPNs detectable?  Yes.  Can malicious actors thwart detection?  Yes.  Can any site really block a user on a VPN?  Not if the user knows what they are doing.