This basically stops the game from reading the file that pollutes your game with the genocide route. This is not about removing all traces however it achieves the end goal of wanting a new slate.

Pretty much making Chara blind to your previous genocide runs no matter what you have done.

• Press WINDOWS KEY.
  
• Type in "run".
  
• Search "appdata".
  
• Browse to "local".
  
• Browse to "UNDERTALE".
  
• RIGHT CLICK "system_information_963" for drop down menu.
  
• Select "Properties".
  
• Select "Security Tab".
  
• Select Username.
  
• Select "Edit..."
  
• Set Read permission to Deny.
  
• Select Administrator.
  
• Set Read permission to Deny.
  
• Apply.

I do not have a MAC myself however this is the method believed by many to be the working method. If I could get a confirmation that this solved the issue, that would be great.

• Open your filesystem.
  
• Change directory to "~/Library/Application Support/com.tobyfox.undertale/".
  
• Select "system_information_963".
  
• Right Click for drop down menu.
  
• Select "Get Info"
  
• Select "Sharing and Permissions"
  
• Click on the lock icon at the bottom right of the window.
  
• Enter your administrator password.
  
• Click Ok.
  
• Select adminstrator user.
  
• Expand on the drop down arrow and deselect "Read only".
  
• Select your username.
  
• Expand on the drop down arrow and deselect "Read only".

This should prevent the game from reading the file to see if you have given your soul to Chara thus allowing you to do the true pacifist route OR if you want to bypass the nothingness that awaits you. I've tested this myself and it works as I did a true pacifist after 2 genocide runs and I did not get the genocide ending for pacifist. The steam cloud cannot alter the permissions for the file so it becomes unable to punish you for your sins because Chara can't read the file. This is just a quick fix if you cannot be bothered removing every trace of it. A nice simple fix to remove your sins for us lazy people.

Yeah, I know this defeats the purpose of what it is wanting to accomplish in the message etc.. This is for people who want to let others have a turn or just for those who wanted to get all endings but not pollute the others.

Just don't do it again, unless you want to have a bad time!