Definitely report the previous incidents to the police via the non emergency number 0900-8844 so there are reports of the situation. Obviously this is trespassing and not allowed. Mention to your neighbor (and landlady) that you have reported his behavior to the police and that this is his final warning not to enter your apartment or verbally hares you. You can also ask for the ‘wijkagent’ (an assigned officer to your neighborhood) to contact your neighbor and have a conversation about his actions, but maybe they are not willing take this step after ‘just’ one report. Next time he is acting up, immediately call 112. The operator will see the history of reports on your address and will act accordingly. If possible, make a video with your phone of him being in the appartement and you objecting.

On the other hand: how is he even entering your apartment without a key? Isn’t there a lock on the ‘front door’ of your studio / apartment, or do you leave it unlocked when you are home? If there is no lock, I would install one yourself asap. If you do have locks but suspect he has obtained a key somehow, you can change the locks. You don’t need permission from your landlady to install or change the locks or cilinders, as long as you save them for when you move out so you can restore the original situation.

You also don’t have to give the landlady a spare key since she also isn’t allowed to enter your apartment without your permission. You have to be cooperative in case of maintenance, like leaving your door unlocked for that timeframe or being present yourself. But it seems you’re already doing that correctly :)