There isn't a reliable way to achieve this.
- If you log the IP address used to download a coupon you are locking out people who use the internet from behind some sort of proxy. This would include people visiting your site from their place of work, an internet cafe or their school. Also people with dynamic IP addresses would be able to get multiple coupons.
- If you require people to register they can use multiple e-mail addresses to gain multiple coupons.
Your best bet might be to simply state in the terms and conditions that only one coupon per household is allowed and rely on the companies who redeem the vouchers to spot if the same people are coming back multiple times.
Most people probably won't download more than one coupon anyway, and people who are determined to abuse the system will try to do so regardless of what barriers you put up. You'd be better off investing the time and money in other areas to reduce the likelihood of abuse.
Having said that requiring registration might be a cost effective approach. It would make people think about what they are doing and most people probably won't think about using another e-mail address - especially if you e-mail them the coupon rather than having it available to download.