Sketchpad has a keyboard shortcut (ctrl+B) for erasing traces created by an animation. This trick offers a slight improvement, allowing you to include an "erase traces" button in the sketch so that the traces can be removed by clicking with a mouse or stylus rather than by turning to the keyboard. It seems to disrupt the interaction less, especially when Sketchpad is used on an interactive whiteboard or tablet.
To construct the button, I used the fact that any presentation button includes the option of erasing any traces before beginning the presentation. Essentially, the idea shown here is to create an innocuous (and invisible) presentation and erase the traces before beginning that presentation.
Create three points, here they are labeled A, B, and C. Select C, then A and click on the Edit menu --> Action Buttons --> Movement. The Movement Action Button dialogue should appear. The speed can be instant, since the movement is unseen and will not affect anything else in the sketch. This creates a button with the default label "Move C -> A". Create a similar button by clicking on C and then B, and it will be called "Move C -> B".
Now select the two buttons (the order will be of no consequence) and click on the Edit menu --> Action Buttons --> Presentation. Select "Sequentially" and check the "Erase Any Traces" box. Now click on the "Label" tab in the same dialogue to change the name; I have changed this button to read: "Erase Traces".