permissions is the best way (Never do this on your C: drive!)

simply right click your drive (D:*) properties\security tab and select edit then add , now type in your username already created by you for the visitors then click OK, In this dialogue box with the username highlighted you can either edit permissions or you can simply tick deny where deny will override any other permission.

click OK and you get "The files permissions will now be updated. This may take some time."

now login as your "visitors" username and check that has worked ?

Double clicking on the file should produce D*: is not accessible "Access Is denied!"

let us know if this works