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 Edit

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. Click on 'Edit' next to 'Generation' to navigate to the generation infos. At the top, find the Static generation with the left / right buttons and click the + button to add it. Also make sure that no other spawn infos are left in the generation infos ('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.