How to block sunlight from loft windows?

I would buy "frosted" plastic (like Contact paper) that can be installed, yet removed. It lets in dappled light.

I agree with Fiddledd224. Some sort of contact paper that can be easily stuck over the windows to diffuse the light. You may be able to find a dark colored contact paper that will block even more light.

I just read a trick to make clear contact paper frosted! You spray windex in the window prior to putting on the contact paper. Cut the contact paper a bit larger than your window. Use an existing straight edge of the contact paper against one frame of the window, apply and work the air bubbles across until they disappear and trim the excess with a box knife.

Alternatively, if you want them to be totally blacked out, you can either paint the windows (can be removed by scraping off with a straight edge blade), apply decorative fabric with starch, etc.

How high are the windows from the floor?

Are they on the ceiling or on the walls?