The option trash Remove can be used to delete a mailshot. It is available from the menu more.

This option deletes a frame, including the connection to a mailshot. This is not offered after the mailshot has been released.
If the connected mailshot should be kept, the option unlink Disconnect Mailshot  must be used first, then it is possible to delete the empty frame using the option trash Remove.

Only mailshots that are not linked the next step of the campaign can be removed. If the option is not available in the menu, then the next step of the campaign is linked to the mailshot's delivery and it is necessary to gradually remove all subsequent steps.