You can use the option to terminate instances through Workflow Administration Console. Launch the Workflow Administration Console, then go to Workflow Server Node > Advanced Diagnostics > Persisted Instances >select the  [Workflow Name] and expand , then there should be an option to delete those instances. Then you should be able to delete the Workflow rule after that if necessary.

In case the current workflow rule is looping and generating more instances, then the first step to do will be to disable the starting rule through workflow designer. 

In case the issue continue i suggest  contact your re-seller to submit a case with LaserFiche Support Dept. on your behalf.