can stencils crop things?

I want to draw a ring but I dont know how to do it. So I thought like if I draw a full circle and crop the inside, it would be enough for me. Can stencils be used on this? How would you do it?

http://forum.facepunch.com/showthread.php?t=1578187&p=52667091&viewfull=1#post52667091

You can make your own draw.Circle function using

surface.DrawPoly (second example).