There are two ways to stop a campaign for a specific lead:

The first way to stop a campaign for a specific lead is to change the lead status to any status other than the default "Lead" status. By doing so, the sequence for this lead will be stopped automatically so no more emails in this campaign will be sent to this lead.

To change the status of your leads, you can navigate to the Leads section of your campaign, and click on a lead to get the right panel.

If you made a mistake, you could put the status back to "Lead" and this lead will be reintroduced to the sequence and continue from where it got interrupted by the status change.