Q2A core does not have feature to limit tag characters. If you have knowledge of PHP, you can check unjust tag on your own filter plugin.
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.