Sure is.

Go to K2 Management>>WorkFlow Server>>Workflows and find the workflow in question and select it. On the instances tab you can start deleting the ones you want.

Does anyone know how to do this in a bulk fashion to delete 100 test workflow instances for example?

Unfortunatly that is not available at the moment. What you could do is deploy a new version of your workflow and then browse to the management console.

Find the workflow and under versions select the previous version deployed and select delete.

Check the "Delete all historical (log) data for selected versions"

This will delete all Active, Completed and error instances of that specific version.