Hi,
There is no direct way but you can add customization for it. You can customize complete task dashlet code , may be change the query to display only last 10 days workflow .
Secondly , you can create sql query that can delete workflows from database directly. This query you have to customized as per latest instance.
delete from jbpm_log where variableinstance_ IN
(SELECT id_ FROM jbpm_variableinstance a where a.taskinstance_ in (select b.id_ from jbpm_taskinstance b where b.actorid_='xyz'));
delete from jbpm_log where taskinstance_ IN (SELECT id_ FROM jbpm_taskinstance where actorid_='xyz');
delete from jbpm_variableinstance a where a.taskinstance_ in (select b.id_ from jbpm_taskinstance b where b.actorid_='xyz')
delete from jbpm_taskinstance b where b.actorid_='xyz'
Thanks,