If you want a specific structure to spawn only once in a world, welcoming the player into it (or imprisoning him or whatever else you might have in store), you can use the Static Generation Info.
Start by creating a structure, if you haven't already. You can read the Guide: Building a Structure for some help. Since vanilla Minecraft's spawn fuzz radius is 16*16 blocks, make sure it's appropriately large and would be able to confine it completely.
Then, edit the structure's info by running Command:edit (if the structure is exported already) or Command:export (if it is not).
In the Structure GUI[]
First make sure the save directory is set to ACTIVE at the top. This ensures the structure will generate once you're done setting up.
You now need to add a static generation info ('natural' is added by default).
Another GUI will pop up. The options are already ideal for a spawn structure, so you can just click save to finalise your structure.
You can now test generating the structure with the Command:gen.
Addendum: If you want the rest of the mod disabled, keeping just the spawn structure functionality, read Using Recurrent Complex as a Utility.