If workflow X has been run before, don't run it again?

Just set the Start Up Condition 'Start When Items Are Created' to 'Yes' in your Workflow Settings. 

You can reach the Workflow Settings, by clicking the Workflow Settings button on the Workflow Ribbon:

You can then set the option by changing it to Yes: 

By leaving the 'Start when items are modified' set to No, and disabling the Start Manually, and Enable Workflow To Start From The Item Menu options, it will only be ran when an item has been created on the list. 

I hope that this solves your problem. 

*Edit 1*

‌' answer below is actually closer to the mark as I totally missed that bit about "whenever it is ran". If you are you are just worried about running something once when the item is created then this will work just fine, but if you are running it at an unknown time in the future, then you'll going to need to make some type of column to keep track of that status as Eric has suggested. 

I would suggest creating a list column for the list that is a Yes/No and Text type.  You can use this column as a condition on whether or not to run the workflow on the item. Default that column to No. The workflow will only run if the column value is set to No.

Go to the Workflow Settings and choose "Conditional" for the "Start when items are created" and/or "Start when items are modified" settings.  Next set the workflow to run conditionally if the column is set to No and add a step at the end of the workflow to update the column to Yes.  If the column is set to Yes, then the workflow will not run if the item is modified.

While I still have a gripe with Nintex for not including this functionality, your solution should work fine.  Thanks!