Can you knockout specific receptors in an adult?
Short answer: yes, it is possible.

But there are some requirements for your delivery system. "Gene knockout" ("gene KO") means such DNA modification that is permanent (get inherited by cell's progeny) and deleterious to gene function (e.g. protein no longer get produced).

So your system must deliver DNA "mutation agent" to all cells (of organism or small specific area). This can be done by using viruses and, for example, CRISPR/Cas9 system for genome editing. Idea  is that virus will get in the cell, start expressing component for genome editing, multiply and infect neighboring cells. Modification in virus can make it non-lethal and less harmful, so you don't kill tissue in process. Virus delivery can be site-specific (e.g. direct injection into the area), it also might be cell-type specific if virus is tailor accordingly.

Also, remember that CRISPR/Cas9 and other similar techniques have efficiency well below 100% and also that you often have two copies of the same gene. This means that total KO probability goes as square of single copy KO probability. If for one event probability is 50%, for both it will be 25%; 10% becomes 1%.

So, all principle component for such experiment already exist.