The original question(s) looked to me like a more complex problem with more than 2 variables
Pacman or pamac can ignore updates for installed packages but cannot blacklist a package that is not yet installed
Pacman or Pamac should not install packages without warning. They should notify packages to be updated,installed,removed or ignored, and user must agree any changes
The suggestion to create a dummy package is not what was requested by OP, but has been marked as a solution to the underlying problem
libgdata was dropped from Manjaro stable branch 2026-06-26, but the list of package changes is no longer available from the release announcement