No, your neighbor/landlord cannot enter your apt without reasonable notice or an emergency. However, you only say you suspect illegal entry. Unless you are willing to rent security cameras for a month to catch him, you can't really file charges against N or L for illegal entry or sue for abuse of access.
Write L (certified, return receipt) & say that you suspect N is entering your apt without notice at non-emergency times. If you catch N doing this, you'll sue L for abuse of access/trespassing. Next, turn N in to police. Send a copy of your report to L. That way, L can't claim he had 'no idea' that N was a bad person. If the conduct is bad enough, L may be forced to get rid of N or at least fire him. If L makes any effort to get rid of you, sue him for retaliatory eviction.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.
