How to report spam to google

Can someone tell me how to report spam to Google. I have spent a lot of time researching this. So far every thing appears to point to using a gmail account to block it.

However, some people don't have a gmail account (or have both). The abusers are not sending the spam to a gmail account they are sending the spam to other email servers.

For other places that spam my users I send email to abuse@[their domain]. When that doesn't work I black hole their IP and don't get the spam anymore.

For obvious reasons, blackhole'ing google.com and gmail.com isn't an option. Unfortunately I'll have to deal with it.

I'm certain someone might know of some relief since it's so easy to get a gmail account and using it to start spaming people. While I can't find their abuse reporting mechanism, I'm certain one exist.