If you are defending someone trying to dunk and you can barely reach the backboard when you jump is this impossible or can you do something to defend? Or should you throw in the towel and just give up?
There's always going to be someone bigger and more physical than you. Every player in the NBA can dunk, and they still get dunked on pretty much every game - the whole point of a dunk is that it's very hard to defend against once the player and ball are in position.
That doesn't mean you can't defend against it though - but rather than worrying about blocking the dunk itself, focus on stopping the player and ball getting into position in the first place. If you're getting dunked on from passes into the low post, work on preventing that pass. If you're getting dunked on from drives, work on keeping the ballhander at the perimeter and preventing them from driving towards the basket.