There are several ways for a Sequence to end for a contact. They will fall in to one of two categories: Automatically or Manually.
For contacts to automatically have a Sequence ended for them it falls to the settings of the Sequence.
When creating your Sequence you have the option to decide what happens to your contacts that Reply or Reply in the negative.
For someone who just replies to an email, regardless of intent, you can set the reply to knock them out of the Sequence automatically. This will prevent them from receiving further correspondence from this particular sequence. Please note this will not affect them in other sequences they may be enrolled in.
If you decide to only end Sequences for people who reply with negative intent then a reply with a negative sentiment analysis will need to be detected by the Engage AI. Examples of this could be someone responding with a one word reply of "Unsubscribe" or of like ilk.
If you decide that you would like to manually end a Sequence for a record that can be done as well. Simple click on the Contacts link in the table overview for your Sequence. This will take you to a report with all Contacts in the Sequence. If the record you are looking for has not completed the Sequence there will be an End Early option that will show under the Status column. Click the link and you will be prompted with a new window asking if you're certain that you want to end the Sequence. Clicking Ok will then end the Sequence. Please note that the contact cannot be restarted in the Sequence.