If you choose to refuse a deal, you actually state that the deal has not gone through for a specific reason. 

• You can edit those reasons under Settings > Preferences > Manage deal loss reasons. 
• Refused deals are also taken into account in the statistics.

Deleting a deal is irrevocable. 

• You cannot retrieve it.
• It is not taken into account in the statistics. 

Note: It's better to only delete a deal if you're sure that there has been a mistake.