Likes are an interesting feature. They seem to have value to the recipient without any apparent cost to the giver. I suspect economists could write volumes about the issues that creates. Now, I realize it is possible that there are hidden costs in the algorithm, but since the users aren't aware of those, they can spend like they have an unlimited printing press. I've looked at the back end of XenForo and I don't think anything is built in yet to place a cost or limit on likes, but perhaps you can lean on the developers to incorporate something. I feel a simple throttling system will even things out more. For example, new members could have 5 likes to give a day, active members could have 10 likes a day, silvers 20, golds 30; or new members could give 1 per hour, silvers 5 per hour, golds 10 per hour. That will accomplish many things- people will like more judiciously, long term members who understand the character of the site will be free to "like" more than newer members (with the idea that they have a more measured eye), and likes will hold more personal meaning to the recipient.