Although I can't stop the requests from being sent, a standard line I add into all my nginx confs to at least minimize my logfiles:
location = /favicon.ico { access_log off; log_not_found off; }
location = /favicon.ico { access_log off; log_not_found off; }
I add <link rel="icon" href="data:;base64,iVBORw0KGgo="> into my HTML head section. I don't recall offhand if it stops the initial request in all browsers, but it does at least prevent repeated requests.
Most people would just tell you to drop a blank favicon.ico if you really don't want a site icon (and I don't), but I'm not going to clutter up my root directory with junk (this, two Apple icon files, and whatever else people decide to do in the future.)