Illegal construction without minimum distance to neighboring property

huh???? What are you talking about?

Sorry. was meant. I misused the auto-completion of usernames.

Mea Culpa

I had seen it then as well and suspected it, but at first I was still a bit startled...

he accidentally tagged the wrong person....

Yelling is just stupid. Just make it clear that after careful consideration no document will be signed and that's it. There's no need to slam doors, shout, or anything like that. If that’s not enough, you can take further steps. But until then, this should be sufficient.
The other party doesn't even know yet that the original poster doesn’t want to sign it. He initially asked here for information to get a comprehensive picture.

...
Well, whatever! Next week the vacation is over and on the next call the matter will be settled from my side.

If you want to make it a bit easier for yourself, even if that might be somewhat childish (I sometimes do that truthfully to make life easier for myself; sometimes I even pay a lawyer for advice so I can rely on it), you can also say you have sought legal advice (you don’t have to say from this forum) and therefore unfortunately "cannot" sign. It sounds a bit less harsh than saying you don’t want to, and everyone, in my opinion, must understand that a reasonable person cannot, does not want to, or will not sign anything against legal advice. Maybe your previous neighbor will then also calm down more easily.

If you want to make it a bit easier for yourself, even if that might be rather childish (I sometimes honestly do that to make my life easier; sometimes I even pay a lawyer extra for information so I can refer to it), you can also say you have had legal advice (you don't have to say from this forum) and therefore, unfortunately, "cannot" sign. Sounds a bit less harsh than saying you don't want to, and everyone, in my opinion, should understand that a reasonable person cannot, does not want to, or will not sign anything against legal advice. Maybe your previous neighbor will then also be quieter.

I will probably do that too. The next call from the seller is sure to come

Screaming is just plain stupid. Simply make it clear that after careful consideration no document will be signed, and that’s it. There is no need to slam doors, scream, or whatever. If that’s not enough, further steps can be taken. But until then, this should suffice.

Since the seller only contacts me by phone, I don't think that will be necessary. If it is, I can simply ask him to leave my property and that's that.

Inform the LRA in writing that you will not give consent for the reduced setback of the illegal construction. Send a copy of this to the seller and that’s fine. Why should you take the seller into consideration, after all, this is not about having a good personal relationship with someone who will live next to you for the next 30 years.