This app was mentioned in 3 comments, with an average of 1.00 upvote
Privacystar is amazing. Not only does their app let you block calls by category, but it has a database of known robocallers, and gives you a button to report the call to the FTC.
Easy way to report spammers/telemarketers (through an app or feature built into Phone or SMS app). This page shows two web pages (FCC and FTC) for reporting unsolicited calls or texts (which are illegal to cellphones and phones on the Do Not Call Registry). So make an app that copies the phone number of the most recent incoming call (or sms), opens the relevant webpage, pastes the number and any other preset info, and lets you hit Submit.
Actually the PrivacyStar app looks like it can file FTC complaints...
Ya, I bet someone could write an app to do it. There's already 2 websites to report them (FCC and FTC). Check this page: https://www.fcc.gov/consumers/guides/wireless-phones-and-national-do-not-call-list
Just have an app grab the most recent incoming phone number, open that web page, and paste the number.
edit: Actually the PrivacyStar app looks like it can file FTC complaints... Would be nice if Google implemented a similar feature.