Roommate from Psycholand

Hello Leute, I used to live in a 4 person WG and last year moved out to my new apartment just 2 mins from the WG. Two more moved out as well and just 1 person stayed there, whom I consider and look out as a younger brother. Let's call him IG. Now, he has the main contract of the WG and 3 new joined to fill the place. They all have sub contracts under him. These 3 are Germans. Now, it's 2 girls and 2 boys. So, sometimes I still visit the old WG and hang out with IG or he is sometimes at my place. I have some emotional attachment to the place as well. Now, the new boy, let's call him Boy 1 has a crush to the girl(let's call her Girl 1 and the other girl – Girl2). Girl 1 moved into my old room. Boy 1 said his feeling to Girl 1 and she clearly said in the beginning that she's not interested and not looking for a relationship.
Boy 1 didn't seem to understand this and followed up on this bothering Girl 1. Long story short, he will sit outside her door on the floor and wait for her door to open. He writes her poems and post it on her door. (This one I actually saw when I visited the WG once, as a note with a long poem was hanging on the door). He constantly messages and call her. All this time, she made it clear she's not interested at all. He kept saying she doesn't realize she's in love with him and just confused. Now Girl 1 talked to Girl 2 about this and she was surprised that this was going on this long. Girl 1 moved out to her parents place as she didn't feel safe anymore as he now walks into her room and sits in her bed and talks to her, whenever she opens her room door. Girl 2 bought all this to IG's attention as he had no idea as well all this was happening. They both thought everything was fine as they usually greet each other or meet in the living room to hang out.

Girl 2 talked to Boy 1 to stop all this as he's just making everything worse. Boy 1 in turn told everyone to back off and they all are harassing him both mentally and physically. They shouldn't come between him and Girl 1 and also Girl 2 should see somebody for her mental issues. Now , girl 2 got scared and moved to a friend's place. IG had enough and asked my opinion and I told him to give Boy 1 notice period and kick him out, as he has become a nuisance to others. They talked with him and Boy 1 said that he will move out. But it's been weeks and he's still there. They already gave him a proper document of his termination of sub contract. Now he's just making excuses that he still has contact and they can't kick him out or this is Germany and he has rights.

With both Girls out his attention is at IG and now he views him as a obstacle to fulfilling his true love. Because of him the girl is scared and all. He gave IG this letter as well, which I posted with this. So if someone, dealt with this kinda situation ,please help out. Involving police or a lawyer seems to be a lengthy process. Also, the landlord is won't be a big help as he never really comes to the place and we maintain the whole building by ourselves. He just inherited the place. So if someone has any ideas, it will be great.