In the KNIME WebPortal, right clicking on a Workflow gives the option of "Discard All Non-Running Jobs" which is really useful. Please can this feature be added into the KNIME client also be right clicking on the workflow within the KNIME Explorer panel. Doing one at a time in this manner is rather tedious.
selecting multiple jobs at once (e.g. via ctrl+a) makes deletion less tedious but I agree in general, that a more comfortable solution for the whole server is needed (e.g. keep max of number X/space X/days X jobs).
For now I have to check manually or run scripts (see http://tech.knime.org/forum/server/cleaning-up-the-server)