Hiring is difficult, and sometimes organizations make mistakes. In the best case, the employee quickly realizes that the job is not for them, and they resign to find a better opportunity. In the worst case, the employee lingers, failing to achieve in their role and even making it difficult for other employees to get their work done.
A bad hire might even be worse than unproductive—they could have a toxic effect on the workplace, bringing strife and conflict into what was formerly a harmonious environment. Whatever specific effects a bad hire has on your workplace, the best option is to act quickly and use formal coaching and performance management to try to correct the issues. If this fails, or if escalation is warranted due to their behaviour, enact progressive discipline. The longer you wait, the more damage the bad hire could do.