This document guides you through a simple method to avoid recruiting MTurk workers for your studies who already participated in a certain study of yours. The core of the procedure relies on Excel (as opposed to CLT or the MTurk API) to assign a Qualification to multiple workers at the same time. Using this procedure will allow you to exclude from the recruitment workers who participated in a previous related study (e.g., a study you are now replicating), and can be functional to other goals too (e.g., executing longitudinal research, building your own panel).
Update June 10, 2015: Arnoud Plantinga developed an script on R based on this method in which you don’t have to create the new variables yourself. You can download the script here.
Experimental Turk 
Thank you for posting this! It really helped me out!
By: A on February 23, 2015
at 11:12 pm
Turkprime.com gives you greater control over your Mechanical Turk HITs. If you create your HITs through TurkPrime you can easily include/exclude people who took a previous study.
By: TurkPrime on April 22, 2015
at 12:08 pm
This may be of interest to your readers:
http://blog.turkprime.com/2015/05/exclude-workers-with-one-click.html
By: Dr. J Robinson on May 7, 2015
at 8:01 pm
[…] MTurk API or GUI allows you to create qualifications that filter out unwanted […]
By: Using Nonnaive Participants Can Reduce Effect Sizes | Experimental Turk on June 16, 2015
at 12:22 pm