There is a way to hide the dashboard prompts in OBIEE.
But, need to have a separate section containing this prompt.. that’s reason i made my article title as: prompt/Section 
Here is the way..
1. Edit Dashboard, go to Properties of Section > Format Section..
2. Expand Custom CSS Style Options (HTML Only) then select Use Custom CSS Style check box… and write the following code as shown:
display:none;
It’s more effective then collapsible option..
