Hi,
Do you intend to block the port 1723 inside VPN connection? If yes, here is the thing you need to configure.
router(config)#access-list TCP deny x.x.x.x y.y.y.y a.a.a.a b.b.b.b eq 1723
Then,place the acl to the appropriate interface of the device.
Hope it helps.
--Jaffer