There is a menu command to remove them, "Restore Original Volume". It's under Tools.
You can always choose which files to apply them to, or not. If they're being applied automatically, turn off "calculate and write missing replay gain tags for new music files" in Tags (2) preferences. I think you could write them manually, but why would you want to? The point is that the computer tries to figure out what is needed to get everything the same volume. For manual adjustment, there's a slider on the Settings tab of the tag editor that's more suited for that purpose.