Can anyone explain how after [Noah's] flood there was still nephilim in the land? Were't they destroyed?

That was only a description of what the spies thought the people looked like. It was an exaggeration intended to scare the Israelites into not going against those people. There were also tall people then just as there are now. But these were not the original Nephilim. The Nephilim were not destroyed during the flood but gave up their fleshly bodies and returned to heaven. But, God forbid them to take on human fleshly bodies again. So the people the spies saw were just large humans. Look at Goliath. He was not a Nephilim.