If your results have not yet been provided to a study, yes, you may withdraw your agreement.
However, please note that when your results are included in a scientific study, your results are uploaded to the NCBI GenBank database as supporting data. Once they are added to the NCBI Genbank database, they may not be readily removed. To do so, you will need to contact NCBI Genbank staff.
In addition, although NCBI Genbank staff will make every effort to remove your results, the generated ID will remain in the database, and your results may become part of third party websites beyond the control of Family Tree DNA, the original research team, and NCBI Genbank staff.