You can cancel a campaign even if it has already started messaging your contacts. Canceling a campaign will stop your campaign message from being sent to any more contacts. However, after a contact receives a message, there is no way to unsend it.
Only the user who created the campaign or an org admin can cancel it. Users who are not org admins will not see the option to cancel the campaign if they did not create it.
How to cancel a campaign
Go to the Campaigns tab.
Locate the campaign within your list of campaigns, then click to view the campaign.
In the top right corner, click Cancel Campaign.
Tip: To check which contacts have already received the campaign message, go to your campaign results then view the Delivered folder.