Linksys router question?

Someone around my house keeps hacking into my router and stealing my internet we aren't happy because we pay for our internet $45.00 a month and there not having to pay for it because there getting it from us free we don't know who it is and cant seem to find out who it is all it says is DonnaPc and we have changed out password a few times and still they hack into it ,after we change it ,My mcafee shows up our name and for our network then 2 hours later she has hacked back into it and it changes back to DonnaPc network how can we block her from tapping into our network or how can we find out who it is any help would be great thank you