It is not possible to disable paste on middle click. Even if you uncheck the respective option in System Settings > General Behavior and reboot, this behavior remains Operating System: Kubuntu 25.04 KDE Plasma Version: 6.3.4 KDE Frameworks Version: 6.12.0 Qt Version: 6.8.3 Kernel Version: 6.14.0-15-generic (64-bit) Graphics Platform: Wayland
Already reported as Bug 505749