Dithionite quenches NBD by chemically reducing the nitro group to an amino group. There are several noncovalent ways of quenching fluorescent dyes, including collisional quenching (with substances such as iodide and acrylamide), static quenching (with antibodies, for example), and resonance energy transfer (with nonfluorescent dyes). The fluorophore can also self-quench when present at a high local concentration.
Since you are using a lipidic derivative of rhodamine B, you might consider incorporating a lipidic derivative of a suitable dye quencher into the same environment.