Since Ubuntu Dock is a system extension (package name:
gnome-shell-extension-ubuntu-dock), it is not possible to simply disable it using GNOME Tweaks or from here.
Removing this package by running
sudo apt remove gnome-shell-extension-ubuntu-dock
should get rid of the dock, but it is not at all recommended as it will also remove the
ubuntu-desktop meta-package which may break stuffs later (see this).
Arguably a cleaner solution is to use vanilla GNOME (GNOME shell sans modifications made by Ubuntu). In vanilla GNOME, Ubuntu Dock disabled by default.
You may install vanilla GNOME by running
sudo apt install vanilla-gnome-desktop
Once it is installed, reboot (or re-login). Once you get to the GDM login screen you should find a cogwheel (⚙️) next to the sign in button. If you click on the cogwheel you should find options titled "GNOME" and "GNOME on Wayland" along with Ubuntu and Ubuntu on Wayland. Select any of the GNOME options instead of Ubuntu options.