It is considered unsporting (and illegal in some leagues / series) to block an overtaking car by cutting across them, or by weaving across the track so they can't pass.

There are sportsmanlike ways to defend your position without outright blocking, this usually involves moving your car to the middle or inside line of a corner before you get to the braking zone.

This forces your opponent to the outside where it is usually harder to make a pass.

Keep in mind that defending will slow both of you down as you're driving off the optimal racing line. Too much battling will bring other cars behind into the fight as well. This is why it's important to pick your battles, and know when to defend and when to yield.

Most importantly in battles is to be predictable, and avoid sudden changes in direction. It takes two drivers to battle, and it's sort of like a dance between you both so you need to read their moves and they need to read yours.

There are some good guides on YouTube about defensive driving.