Few things you could do to minimize the contact page abuse.
1. Change the link to the contact us page to a new one, so spiders won't have it on index.
2. Make the contact page an orphan one, meaning spiders won't have a direct way from the domain root to find it. Humans can still access it as before(you could use js or a form instead of displaying a direct link).
Spammers locate contact forms from the SEs and these pages have no value in terms of SEO. However if you have a physical presence for your business and you need spiders to index location/phone details use a separate page. Or you could structure it so the form is shown with a js popup once a selector is clicked (not a hard-link) so human visitors see all contact details and the form in the same page.
You could also limit the number of automatic submissions by using cookies, css and/or js. For instance you could check if a session cookie exists before allowing the form to be processed.
Now if the mail spam comes directly to your email address (not via the contact form) is a different matter. I use a helpdesk to retrieve and process my business email, use your domain's mail accounts so you have complete freedom to see all mail headers and filter them accordingly.
PS: Regarding spam from India, I get spam emails from there too. But I get spam mail from every country. It's not country related. I also get phone-calls from people from India, offering projects to work on, or exchange work. So it's unwise to block an entire country's IP range. Spammers operate via systems in every nation.
[edited by: enigma1 at 10:41 am (utc) on Jan 21, 2012]
