Elasticsearch sets a default limit of 10000 for the search.max_buckets setting. This can be changed, but it can also have a detrimental effect on the cluster, as you say if someone sends "killer queries".
Kibana has a default limit of 2000 for the max buckets in the Advanced Settings under Management. This is a conservative limit and can be set higher. But passing a lot of data around in the browser can be extremely resource intensive and cause browser hangups.
Perhaps we should try to optimize your visualization rather than change cluster settings. Which visualization are you using that needs so many buckets? Does it make sense to visualize that much data at one time? There is a limit to how much detail human eye can perceive.