There is lots of ways to stop bots. They could just monitor how often a person uses the same welcome message but then they could mix up their messege.

They could require all curation votes that get a payout to have at least 1 vote that is not themselves.

Add a report bot function.

Flag people that first vote early every time and audit the largest offenders. This would require admin but if you penalised those found to be bots by fining them with steem or warning of account closure they might stop.