Technically defending the inside line and pushing someone onto the rumble strips is illegal as that means they are exiting the white lines and therefore you are pushing them off the track...
However the other view is, you have the line, they should yeild until they can actually overtake...
You should only make one move, and any faster driver can easily make 2 moves to overtake, not just brake later than you
Personally, on a track, 1 blocking move is fine by me as long as there is no contact and you yeild as they pass you
Which by the sounds of it you are more than willing to do
This makes good racing and very interesting to drive, and enertaining to watch!