This prints to stdout the process ids of everything running on <port_number>:
fuser -n tcp <port_number>
It also prints some stuff to stderr, so:
fuser -n tcp <port_number> 2> /dev/null
We can then supply these process ids to the kill command:
sudo kill $(fuser -n tcp <port_number> 2> /dev/null)
You could also put this in a function if you do it a lot:
function killport() {
sudo kill $(fuser -n tcp $1 2> /dev/null)
}