Generally speaking, a landlord may enter the premises only to make "necessary or agreed-upon repairs," and even then the tenant must be given advance notice in most cases.
You do have legal recourse, as this is a statutory right and the law provides monetary penalties that you can recover for each incident of unlawful entry. Try writing a letter to your landlord first, explaining that you won't tolerate any further unlawful entry and will pursue legal action if it continues. If that doesn't work, you could file a complaint against your landlord in small claims court. You wouldn't be entltled to a lot of money, but it might get his attention.