It will be less likely that they'll win because they gave it as a gift, which according to paypal means there were no goods exchanged. I'm not sure how to prevent it, maybe someone with more experience with paypal will post.
Yes you can and there is no 100% way to prevent it. Best bet is to refuse anyone from paying via credit card (have them screenshot the payment details page after they send it and look at the line that says "funding source". Make sure it says bank or balance)