I'm trying to restrict VPN usage on my router and only allow my corporate VPN FortiClient SSL-VPN, while blocking all other VPN connections. Is it possible?
The FortiClient VPN works fine.
Router: TP-Link Archer AX23
Version: OpenWrt 25.12.2 (r32802-f505120278)
Research BanIP
There are lists with IP addresses of VPN service on the internet which you could use
What exactly are you trying to do? Those specific kinds of corporate VPN solutions are designed to look like HTTPS traffic, and they are very common. There's Cisco AnyConnect, Palo Alto GlobalProtect, Ivanti Connect Secure, Microsoft SSTP, to only name a few.
If you want to prevent your children from using VPN services that commercially advertise privacy but in reality specifically cater to a video-streaming audience who want to circumvent geo-fencing: There's probably lists of IP addresses out there.
But if you're e.g. a hotel and want to prevent your guests from doing anything except browsing, there's only so much you can do.