Getting rid of a roommate can be a simple process if he is not on the lease or is causing trouble. If the roommate is not on the lease, he has no legal right to live in that location and can be kicked out immediately.
A roommate that is not on the lease can be kicked out for whatever reason by the person who is legally liable for that lease. This means that the roommate does not have a legally binding contract to live where he does, and the person who is currently signed to the lease has the power to evict as he desires. When the roommate is on the lease and causing trouble, then the landlord should be contacted.
The healthiest way to get rid of a roommate is to have a conversation with him in an adult manner. Explain to him what is going wrong and why he is being kicked out. Avoid accusing him of specific wrongdoings.
Keep the conversation professional and comfortable. Do not make it personal, and avoid attacks on his character. Let him know that the situation is not working and that he should find another place to live. If all else fails, the landlord can be contacted to get involved in the matter.