So far as I know, you cannot remove the spawn radius, you can only lower it to "/gamerule spawnradius 0" so that all players spawn on one specific block, or raise it with "/gamerule spawnradius 100". The only way that I am aware of to change where the spawn radius begins is to stand on the block where you want the circle to start and then enter the command "/setworldspawn". Sorry if this isn't a great explanation its been a while since I have made a server. :')