Hi Thangam,
You can find the "Advance with immediate dialogue" in the "Details" tab of the step. Check for a background task which gets triggered after your decision step and deactivate the "Advance with immediate Dialogue".
http://help.sap.com/saphelp_nw70/helpdata/en/8d/25f5a6454311d189430000e829fbbd/content.htm
You can also deactivate at the header level. If done, you cannot active the feature in any of the steps of that workflow template.
Thanks,
Karthikeyan