Removing tags either conditionally or in every case

Hello there, I realise that I could filter and use extended tags to get rid of things that are unwanted, but that is annoying when I suspect that there is an action which can be created to do the same thing in one step.

Let's say that I have a lot of files and they were bought from Itunes. I want to create an action, which detects and deleted all instances of Itunes as a source, whilst ignoring all other sources and keeping them in place. How can this be achieved with an action?

In another scenario, I may decide that I don't want the source tag at all in a selection of files, how would I delete the source tag for every instance the tag is detected using an action?

Thanks for your help.

see the help on actions
in your case could be helpful

or you replace the pattern in a field so that the field is not filled anymore. Empty fields get removed.

Thanks for your reply. This did help in some instances.