It is not clear from your question, cebn04 what stage the prosecution has reached.
The previous answers cover circumstances where a conviction has not been recorded. That is, the defendant has pleaded not guilty and a trial is still to come. However, your question seems to suggest that only sentencing remains (either because a guilty plea has been entered, or the defendant has been found guilty at trial).
If this is the case, you cannot stop the process whatever you do. A conviction has been recorded and it stands whether you withdraw your complaint or not. The only influence you may have over sentencing is by completing a �Victim Impact Statement�. This can be completed by all victims and is read by the sentencing magistrates (or judge in a Crown Court). It is usually done to impress upon those passing sentence that the offence adversely affected the victim�s life. You may complete one to say that, despite the assault, all is now hunky dory.