There will be more complaints from the users if Stake add captcha for the bonus claim. Because the captcha system doesn't work occasionally and considers the solved captcha as wrong even if you do it perfectly. Users will become more crazy in such situations. The best thing Stake can do is to apply the bonus claim limit to its users. If they give 50 bonus codes in a week, then they have to apply 15 times bonus code claim limit to each user account. It will increase the opportunity for other users to claim the bonuses.