Step 1 – Check how messages are scheduled -:

1. Via Workflow rule or Process Builder
2. Via SMS-Magic Scheduler Addon or Via Schedule SMS button option from the object (from record/ list view/ campaign/Converse App)
3. Via Campaign Launcher.
4. Via Event Reminder Add-on

Step 2 – How to remove action based upon the source from where they are scheduled.

How to stop the scheduled actions/messages in salesforce

When the Workflow rule is Active:

When the Workflow rule is Active, the time trigger actions will be triggered based on the criteria. It means, as long as the workflow rule is active, the criteria will be re-calculated and time trigger actions will be fired accordingly. 

When the Workflow rule is Inactive:

If you deactivate the workflow rule, the scheduled actions will not be re-calculated due to which scheduled actions/SMS will be triggered even if the Process Builder is Inactive (As per the default behavior of salesforce). In this case, in order to stop the scheduled actions/SMS you need to delete the scheduled tasks. 

To do so follow the steps below: 

For Salesforce:

1. Go to setup, then search for “flows” in the quick find box.
2. Check and confirm if the listview is selected as “scheduled”.
3. Delete the scheduled tasks which you want to.

For Workflow Rule:

1. Go to setup search for “Time-Based Workflow” in the quick find box.
2. Enter the criteria to monitor the queue.
3. Delete the time-triggered action.

Below is the link for reference:
https://help.salesforce.com/articleView?id=workflow_queue.htm&type=0

When the Workflow rule is Deleted:

If you want to delete the Time based workflow if there is already an action scheduled in Salesforce then you need to remove all scheduled action from Time-Based Workflow (Monitor) queue. Then you can deactivate or delete the workflow.

In short, Even if the workflow is deleted, you still need to remove or delete time-based actions from the Time-Based Workflow queue. If you don’t delete these actions, then these scheduled actions will be triggered accordingly.

To delete scheduled actions from the workflow queue following are the steps:

1. Go to setup search for “Time-Based Workflow” in the quick find box.
2. Enter the criteria to monitor the queue.
3. Delete the time-triggered action which you want to.

“For processes that are set to run when a record is created or edited, scheduled actions remain in the queue only as long as the criteria for those actions are still valid. If a record no longer matches the criteria, Salesforce removes the scheduled actions for that record from the queue.”

Points to be noted:

1. All the above scenarios are applied for both workflow rule and process builder.
2. If the evaluation criteria are set to Evaluate the rule when a record is: created, the workflow rule evaluates its time triggers only once. If the record that fired the rule changes to no longer meet the evaluation criteria, Salesforce removes the pending actions from the queue and never reapplies the rule to the record.
3. Time-dependent actions can automatically be queued again if the record is updated and the evaluation criteria are set to Evaluate the rule when a record is: created, and any time it’s edited to subsequently meet criteria

You can refer below links for more detailed clarification.

FAQ: https://help.salesforce.com/articleView?id=000325723&language=en_US&type=1&mode=1

Considerations for Time-Dependent Actions and Time Triggers:
https://help.salesforce.com/articleView?id=workflow_time_action_considerations.htm&type=5

Via SMS-Magic Scheduler Addon on Via Schedule SMS button option from the object (from record/ list view/ campaign/Converse App)

To stop scheduled SMS with the above ways, below are the steps:

1. Go to the “SMS scheduled” tab.
2. Select and open the scheduled SMS record which you want to stop.
3. Look for the “Schedule JobId” field and copy the value.
4. Go to setup and type “scheduled jobs” in the quick find box and click on the same from the results.
5. Search for Schedule JobId which you have copied from step 3.
6. Select the record and click delete.

How to stop scheduled SMS via Campaign Launcher

1. While you are still on the “Campaign Manager” tab, you should be able to view the List of scheduled campaigns.
2. If not, click on the “Campaign Manager” tab under the “Campaign Launcher” app.
3. From the list of scheduled campaigns, click on the dropdown icon on the right side of the campaign that you want to delete or stop.
4. Select the “Pause Campaign” option. A confirmation selection pop-up will occur.
5. Click on “Yes” to confirm and pause the campaign.
6. The campaign status should be changed to paused.