I can't think of a drug that selectively reduces serotonin release, although such a thing should be possible. However, if the goal is to reduce activation of serotonin receptors, the more accessible approach might be to inhibit the production of serotonin (such as with a tryptophan 5-hydroxylase inhibitor or low-tryptophan diet.)
HOWEVER, such interventions are unlikely to be practical. The brain needs a certain level of serotonin activity to function normally; deny it that and you're likely to get all manner of interesting 'withdrawal' symptoms (depression, anxiety, sleep disturbances, etc.)
A more direct (but at this point rather theoretical) method to reverse downregulation might be to create a substance that increased the transcription of the gene that creates the desired receptors, although you might run up against a forest of problems (activating the wrong genes, actually getting it into the brain, etc.) If such a substance could be produced, it might make an effective anti-depressant.
