I use this to clean up my development database of all process instances.
delete from jbpm_comment;
delete from jbpm_variableinstance;
delete from jbpm_tokenvariablemap;
delete from jbpm_moduleinstance;
delete from jbpm_log;
delete from jbpm_token;
delete from jbpm_processinstance;
This doesn't answer your question, I know, but it is how I keep things clean so the failed experiments don't create too much noise.
No there is no api (yet) nor a ui for this. The webapp can be easily extended in the admin part (currently there is nothing). Develop a page (jsf based), develop a backingbean and implement a simple hibernate query that deletes the pi's. I currently do not have the time to show you, but it should not take more than an hour to build something like this. I can give you my bank account number for the weekend however :-)