Minecraft has a default setting that blocks all non-OPs from breaking or placing blocks within 16 blocks any direction from the spawn point.
This is defined by the following line in server.properties (available under Profiles -> Configs):
Where 16 is the number of blocks, in a radius.
You can increase or decrease this radius as you see fit by altering the number - it will take effect the next time the server is restarted, so for immediate effect use the "Save & Apply" button in ClanForge.
Removing Spawn Protection
To remove the spawn protection entirely, set the number to 0: