Firing people can be a vary emotional and challenging process.
First though, to be absolutely clear, what an employee does with their own personal time is not a consideration, even if it is binge drinking.
Also, it might be time just to ask them to leave, or fire them without cause, which most companies can do as long as they state it and give full references to the person that are glowing.
You can however document and give them reprimands for poor performance at work or being absent. For instance, being late to work and documenting it, then discussing it with them with a warning, and if it continues docking the related pay, etc. Be warned though, in doing this activity, it is very time consuming, and you have to give the opportunity for improvement and do so in good faith. In performance is improved and meets minimum standards, you would have no choice but to keep them.
If there is a non-disclosure agreement for work that your employee has signed, then the NDA has a built in penalty clause as well. However, you will need someone to verify that a disclosure has taken place. Most NDAs though specify full termination and a clause with financial penalties if working for the competition