There are already countless services for doing throwaway email...
Also, a trick I use is to create a wildcard subdomain:
*.mydomain.com MX mail.mydomain.com
and then when signing up for a site, use an appropriate subdomain, eg:
That way, not only can i stop the flow of unwanted emails should they become a nuisance, but i can also see if someone has sold out or leaked my address to spammers (which has happened a few times)...
When i've had enough of the junk from a particular source, i can just create an MX record specifically for that subdomain which will override the wildcard. I tend to redirect junk back to where it came from.