Failed to start workflow. Could not allocate space for object

We have a problem with our Nintext workflow DB because of suddenly growing database.

It is giving an error message "Failed to start workflow. Could not allocate space for object 'dbo.WorkflowLog'.'PK_WorkflowLog' in database 'NW2010DB' because the'PRIMARY' filegroup is full. Create disk space by deleting unneeded files, dropping objects in the filegroup".

We tried to handle with NWAdmin.exe and tried to clear workflow data and workflow history data but nothing much help. It resolved the problem temporary and again came back same problem. There are workflow instance between 500- 600 in progress which might hold workflow data and workflow history data.

We have created this workflow just month and half ago and started giving this problem.

Any suggestion or help really appreciated.

Thanks,

pdhameliya