@Tenou
They can only use an address within the tunnel range. So, you write your rules accordingly. If needed, you can even restrict the address range by using a longer subnet mask, to the point where there's only one address that will work. Also, if you're worried about that sort of thing, then you should be implementing other security beyond just VPN addresses. For example, if you're on a corporate network, you might be using Active Directory or similar to restrict what users can access.