Hi,

you mean reminders for workflow tasks?

Because if is the workflow already "running", then you are not able to change activities in the workflow.

Hi

Yes I mean reminders for workflow tasks that are active

Then maybe the fastes solution is to cancel running workflows, change "reminder behavior" in WF tasks, publish new version of WF a manually run WFs on required items.

Complete the task.   There's not a way to mute reminders for an active workflow.  You either change the workflow as ‌ suggested, or the end user can create a rule to automatically delete them (which defeats the purpose of a reminder) when they arrive. 

You can also take over a task if the person assigned is not going to complete it, or delegate the task. Then complete the task itself.

Stopping the workflow is also another means, or like ‌ mentioned you can change the workflow for next time. 

Let us know what you think from the feedback and if there is a working answer for you.