Hello There,
Sorry, this is not possible with Discovery Accelerator. Search results once accepted will always remain there in Discovery Accelerator Database. The only way to delete results is to delete the case itself but that will also remove every activity that has been performed on the case.
Specially for the situation when you don't know the precise search crieteria and you might need to delete the search result, you run Discovery Accelerator searches without selecting option "Automatically Accept Search Results". Just don't accept search results till the time you are not very sure about the crieteria and associated result.
Also you can create a research folder in context of the case, where you can run searches per your wish. After those searches, select the Search which has the needed crieteria and run it again in your DA Case. After all the homework, delete your Research Folder.
Cheers
Alok