There are a number of problems with it. First, it won't actually affect spammers unless nearly everyone is using it... what could force them to accept the char

There are a number of problems with it. First, it won't actually affect spammers unless nearly everyone is using it... what could force them to accept the charges? Second, it's turned out that even the email systems that ask people to confirm they sent something (which spambots can't understand in general) are too much for many people to bother with, so asking them to join a system in order to pay a fraction of a cent for each email they write would likely be too much trouble. Lastly, spam filters actually work really well. I get roughly a thousand spams a day to my primary address which has been continuously in use for ~7 years, and have to "mark as junk" maybe two spams a day -- the rest go straight to my junk folder.