How do I stop people spinning the wheel over and over for better coupons?

Use these two settings together in Plugins → Spin Wheel (edit the campaign):

• Require email to spin: shoppers must enter a valid email before spinning.
• Max spins per email: how many spins each email gets for this campaign. The default is 1.

Once an email reaches the limit, the shopper sees "You have already spun for this offer." The limit is counted per email, so if you untick Require email to spin, people who skip the email can't be limited.

Ticking Email the coupon on win also sends the code to that email, which encourages people to enter a real address.