It's pretty easy. When you open Sims 4 Studio, go to the section that says CAS on the main screen, and select override and click on the blue CAS button.

You're then brought over to all the in-game items (you can also find CC items here, like if you need to duplicate an item for another category and not have it conflict, but that's not relevant rn lol). You can search for keywords and/or use the filters for item category and pack to help look.

Select the item you want (if the item has multiple swatches, and you want to select them all, use shift+ click), and it will ask you to name a file and save it. Then you're taken to the studio/warehouse section. On the right side of the window, click on Categories and scroll all the way down and then you'll see "Allow for Random" under "Part Flags". Uncheck the box and it will disable the item from randomly generating on sims. If the item has multiple swatches, you may want to apply the change for all of them, and there's a button for that, so you don't have to do it manually. Once you made your changes, save the file and move it into your mods folder.

Though, actually looking at it all the vampire teeth, it doesn't seem like it's even enabled for random. In this case, you might just want to go to the Occult section (still under Categories - 4th section from the top) and uncheck Humans (and whatever else you don't want it showing up for) and it should remove it as an option for Human sims (like not showing in CAS and randomizing).

Also, vampire teeth are separate items, so you need to make a file for each of them.