If you have access to the database, you would need to look in the tables:
You will need to select the jobs that match the proper class name from QRTZ_DYNAMIC_JOB_DETAILS -
Select * from QRTZ_DYNAMIC_JOB_DETAILS where JOB_CLASS_NAME = 'the class name of the job you want to nuke'
Keep track of the "JOB_NAME' and then delete everything from QRTZ_DYNAMIC_TRIGGERS where JOB_NAME = all of the job name colums you selected in the previous query. Then you can delete from the job details table...
So something like this - * I did not test this code.
DECLARE @JOB_NAME NVARCHAR(200)
WHILE(SELECT * FROM QRTZ_DYNAMIC_JOB_DETAILS WHERE JOB_CLASS_NAME = 'MyNamespace.MyClass, MyDll')
SET @JOB_NAME = (Select top 1 FROM QRTZ_DYNAMIC_JOB_DETAILS WHERE JOB_CLASS_NAME = 'MyNamespace.MyClass, MyDll')
DELETE FROM QRTZ_DYNAMIC_TRIGGERS WHERE JOB_NAME = @JOB_NAME
DELETE FROM QRTZ_DYNAMIC_JOB_DETAILS WHERE JOB_NAME = @JOB_NAME