Is it not possible? 

Hi.

Go to Manage Workflow Inventory under the site actions gear.

Click the Delete icon for that workflow

According to the dialog box.  this will terminate all running instances.

@Usama was your question answered?

You should be able to delete the specific workflow in question by going to your site and adding the following in the URL bar next to your specific site / site collection,

/_layouts/15/NintexWorkflow/NintexForSharePoint.aspx

If this does not work, try going into Site Settings -> Nintex Workflow -> Workflows for this Site.

You should be able to manage your Workflows from here.

I just have the above options, I am using SharePoint server 2019. Are you using SharePoint online? 

Yes, accessing directly through URL works for me. Thanks