Although you have the same account but there is always different ip when log in, are you think there is no way they could ban our account just by using multiple ip? What we currently do is just to minimalize what we can. Let me ask you this, when you are getting banned from them, are you going to say that you played by using different ip all the time and that is your intention from the start? I do not think they will give up on this silly thing
Playing with different IP can't be considered as multiple accounts IMO, it's different case while you playing with many accounts on same IP. Maybe they using VPN to speed up their connection while playing on there, that's why they changed their IP. As long as they playing with 1 account only, i don't think that would be a problem for the site.