Hi ElfQueen,
In CAS, click on the Premade Sims button. (6 Sim shadows button) [Lower left of screen.]
To show to your custom Sims click the star * in the right corner of the Sim Bin.
Select the sim you want to delete. Then click the delete button (trash can) down in the lower left, near the "Share" button.
And just like Lisa said. Outside of the game go to your Sims 3 folder and look for the SavedSims folder and find the Sims you want to delete.
> Documents > Electronic Arts > Sims 3 > SavedSims
Either way will delete the Sim person or people from your game.
I hope this helps, and welcome to the forum,
MrsFlynn