Sorry - I cannot see your attachments (blocked for some reason) but here is a path you may try on using regular expressions and collection operations.

In your regular expression action, perform a "Replace text", and replace the individual's user information with "zzzzzzzz" or the like.  Then perform two Collection Operations: 1. Sort and then 2. Pop to remove the "zzzzzz".  Finally, repopulate the Participants field with your collection.

Obviously try this on a demo list first!

@bgarvey Thanks for the quick response. I did try your solution, but it always seems to remove the last person in alphabetical order, even tho its not the one who is suppose to change. I tried the stuff you said plus i added some stuff to make it work ( build string back before the update, that way its not a collection variable that i put in the column (impossible)). 

Can you check my WF and see where i am wrong.

Hmmm - after you perform the sort, can you log the array to history to see why the zzz is not at the bottom (i.e. the last index).  If it goes to the top for some reason, then you should be able to use Collection Action > Remove for the item at index 0.

And you are right!  I have been working with another action (Nintex Sign)  that allows you to populate a person field with a collection.

@bgarvey I think the text replacement doesnt work since it's putting the one name in the center ? his name start with an S.. it does sort the collection. 

Sorry - unfortunately my company is blocking the png file, so am flying a little blind.

Be sure to REPLACE the entire string including any of the 31#; crap that comes along with the person field.

Alternately, see if you can do a reg expression on the string to identify the index of the user entry, then use a collection operation to remove based upon index.  That is probably more error proof anyway.

I did résolve my problem with the index fix ! Thank you !!