My nether portal spawn is in a soul sand valley, is it possible to spawn proof to keep all of the skeletons from spawning and attacking me as soon as I step through?
You have a number of options available to you, all of which are available on the Minecraft Wiki. I've excluded some options as they don't apply here, such as lighting an area up as that does not spawn proof the nether, but the list includes:
- Bottom slabs or other partial blocks like dirt paths
- Transparent blocks like glass or leaves
- Redstone components, including buttons, levers, pressure plates, and all types of rails
Thoroughly coating the area around your portal with one of these options should do the trick. I personally recommend either buttons or dirt paths as they would be the cheapest ways to cover a large area.
You can spawn-proof an area by covering it in non-full blocks. This can be cheap blocks like cobblestone slabs or carpets, or if you really want to show off you can use blocks like enchanting tables if you really wanted to.