Neighbour's cat headbutting our catflap

[Polyethyl]

Any advice on how to stop this?
I have a 5yo female cat. My lovely neighbour has a 1 year old male cat. We live in London, where many homes have small back yards and there are many cats. Ever since her cat reached adulthood he's been trying to claim our house and backyard as his territory. We have a sureflap microchip cat flap. He stares through the transparent flap at all hours. At night he batters himself against the flap trying to get in. (He has his own cat flap on his own home and he isn't locked out.)
Last night I was awoken by the noise at 4.39am and I listened to the continuous bangs and slaps until he was scared away by a neighbour at 5.56am. He really did headbutt our cat flap for nearly an hour and a half. (I have chased him away on previous nights but he's back within 5 mins.) My poor cat hardly goes out unaccompanied any more and has entirely given up hunting.
I could sleep wearing ear defenders.
Or i could ask the neighbour to lock him in at nights. (But he's such an energetic cat he creates a rumpus in her home and she gets no sleep if she does that.)
I also wonder how robust the cat flap is and how long it will last under such a battering.
Is there anything we can do to get this young hooligan to be peaceable at nightime?

Update. Lovely neighbour tried locking her cat in at night. But he bullied her through the night. So she shut him out of her bedroom. So he learnt to open her bedroom door by pulling down the handle. He has now stopped coming to her call and knows not to go home at evening time.
He has also learnt to open our microchip cat flap, by hitting it, catching it on the rebound and lifting it. He comes in our home at all hours.
I'm now in bed, I was reading a book, then enemy cat popped his paws up against my bed and miowed at me. I've chased him out but I don't know why I bothered. He'll be back soon.

Lovely neighbour and I are at a complete loss as to what to do about him. She can't eat in his presence because it's too tiresome to protect her food from being stolen.
He's wormed. He's fed on premium quality food. He's not hungry, he's just a bully.

Block up your catflap. You should have an indoor litter tray anyway. He'll give up and go torture someone else less kind. You can get mesh for Windows as well to stop him getting in.

I remember a time when men were this persistently trying to get into my bedroom.. naughty puss.

Get a super soaker for outside and nerf gun for inside. Be relentless. He will get the message!