How can I cut off the rusted closet bolts holding my toilet?
I would caution against using any bolt cutters on the toilet hold down bolts. The reason being that they typically will apply a significant pressure against the porcelain base of the stool and crack or break it. 

Another thing to consider is that bolt cutters typically have a jaw profile as shown below. With this type of arrangement it is just not possible to get a grip in the small size of the bolt at or below the nut. 



You will be far better off using a mini hand hacksaw as shown in another answer or a Dremel tool with a grinding wheel cutoff blade.



An alternative if you do not have a Dremel tool but do have an oscillating tool is to use a metal cutting blade in that. Note that it is my experience that the typical metal cutting blades for these saws do not last as long as you would hope and they are much more expensive than a few cutoff wheels for a Dremel tool.