removing from nextdns

I'm not entirely sure what you're asking. I assume you're asking if you can disable/remove opendns since  this other person isn't paying for the internet in your apartment. If that's not what you mean, then please clarify

If my assumption is correct and you have access to the router's settings, you can clear out the dns server entries that are pointing to opendns and use NextDns instead. If you don't have access to the router's settings then you will most likely have to set your personal devices to use NextDns if desired/possible. 

Note: Some more locked down devices, like Rokus, will not allow you to set/customize dns as it will just pull the dns set at the router level. However most mobile devices (phones, tablets, etc) and computers (desktops, laptops, etc) will allow you to set/customize dns so you can bypass opendns at the router level but you will have to do this per device