You could also work with cookies and IP tracking.
If the system detects two accounts from the same IP *BAN!*
Same with cookies.


If you'd prefer to keep it simple activity might do it.
Activity that cannot be automated like meaningful posts or written reviewes/articles on a project.

I remember projects where one had to ask questions or answer them in a group to be regarded as 'active' and getting the reward.
So if someone new joined everyone was keen on answering questions. Same with AMAs and interviews.
Was a totally different experience from other groupchats