The following two lines will prompt you for a text string and then will delete the string in Title.

Prompt for Text "Remove the following text from Title" save text to Variable 1
Replace "\v1" in the Title field with ""

Note that the forum auto mailer removes backslash characters. The Variable specification in the Replace statement has a backslash before the 'v1'.

The best way to learn about actions is to read the following help topics on the main help page:

Actions
How Actions Execute
Variables
Escape Sequences

There are also quite a few fairly well documented actions on the Sample Actions web page (accessible from the Help menu).

BTW: Unless you want your action to do more than the above, you can do it completely from within the Multi Field Editor.