You can do this in your User Text Template.
Once you have set up a User Text Template, click Row Contents.
Choose "Stitch template condensed single" from the table and you'll see in the Preview box that the string has a "1" at the end.
Click Edit (on the right side of the screen) and delete the number.
Click Apply & Close, and return to your chart. You will need to tell Stitchmastery you want to use a new text template for the chart you have open - go to Diagram in the main menu bar and click Edit Diagram Properties. Choose "Stitch Library and Text Templates" from the top row of tabs, and choose your new text template from the drop-down menu. When you save that setting, you'll see the written output changes to "k" or "p" for a single stitch.