@Hugo @Ignasi - +1 to this being a bug, not a fixed issue.
Giglioti, this has been discussed many times before eg: https://forums.funcom.com/t/vertical-land-claim/120465
It was stated to be fixed in the 9 July patch.
@Spynosaur_Nicole clarified “The locations fixed are Barrow King, Galaman’s Tomb and Skittering Cavern but there are more locations planned in the future!”
I have just tested this on an official PVE server. I built on the ridge above Skittering Cavern and it despawned the spider boss.
Blocking of this boss may be against the official community guidelines. You could report them if they aren’t willing to budge and allow other people access to this boss.