If the landlord has a lease provision that forbids the tenant from operating a commercial business from the apartment, then the landlord can seek to enforce that provision -- giving the tenant proper notice to cease the violation and starting an eviction action if the tenant does not. If the lease allows for only two vehicles, then the landlord may be able to have any additional vehicles removed -- but the landlord would want to be certain that he's in compliance with the state's towing laws, as the penalty for violating the law can be significant.
If the landlord has not protected himself in the lease, he can end the tenancy at the end of the lease term. If it's a month-to-month tenancy, he can give proper notice to end the tenancy (30 or 60 days, depending on the length of the tenancy). If the tenant is violating local zoning laws that prohibit the business from being operated from a residential area, then the landlord may be able to get the local government to order the business activity to cease.
The landlord should discuss these options with its landlord-tenant lawyer, who may have additional ideas or suggestions.

