How to stop hacking through utility airport

If you are using a AirPort base station you can reset it to factory defaults, then change the default password

How to reset your AirPort base station - Apple Support

EDIT: Bobs suggestion below will work without the need to factory reset, I just tend to be over zealous with these things.

If you are concerned about privacy with your Apple ID you change the password for your Apple ID and do not share it with any other service (e.g. yahoo, facebook, etc.)

https://support.apple.com/en-ca/KM205079

I recommend any and all Apple device(s) logged in now with your ID to log out and then re-authenticate with your ID and new password once it's changed. If your BF is using your ID on devices available to him then he will have access to your two factor codes.

then if you have not done so already enable two factor authentication

Availability of two-factor authentication for Apple ID - Apple Support

How can I stop that?

You don't mention whether you have an Apple wireless router, but I will assume that you do.

Use AirPort Utility to change the password for your wireless network, and give the new password only to the people that you want to be able to connect to your wireless network.....and trust that they won't give it to another person.

I'm not clear on whether another person also has your Apple ID and password, but if you suspect that might be the case, change the password for your Apple ID and don't give it to anyone else.