When you recommend a post, a few things happen:

1. The number of recs the post have received increases and your name gets added to the list of Xangans who recommended the post.
2. Your friends and subs get a notification in their universal inbox saying that you recommended the post.

There is no way to undo the changes to the post itself (1).

However, you can delete the related feed item from your public feed to prevent your friends and subs from getting a notification in their universal inbox about your recommendation. To do so:

• Go to your public feed.
• Find the feed item saying that you recommended the post and click on its "delete" link (at the lower right corner).