You don't even need to put something in the slits to have which-way information. It's sufficient to put the screen, where the particles are observed close enough to the double slits. It's easy to understand within the wave picture, why you don't see interference fringes: Due to Huygens's principle the wave behind the slits is given by the superposition of the spherical waves coming from each point in the slit openings. If you are close enough to the slits these spherical waves don't overlap there, and thus they do not interfere at the observation screen and you have which-way information for each particle. If you put the screen far away from the slits you get the usual Fraunhofer diffraction pattern, because the partial waves from the slits overlap at the screen and thus you can't know from which slit each of the individual particle came. In this sense you have a typical case of what Bohr called "complementarity": You can either observe "particle aspects" or "wave aspects" of the "quanta", depending on the choice of observation (in this case putting the observational screen close to or far from the slits. You can't have both aspects in one experiment, but of course you can put the observational screen somewhere in between. Then you have some "uncertain" which-way information and a somewhat "blurred" diffraction pattern.
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