Configure sharepoint permission with powershell

Hi All,

If you delete the user authority set for the sharepoint site by the following operation,
 The permission of the list that does not inherit the permission will change.

I would like to change the access of the site without affecting the access of the list.
 Could you give me some advice?
 
 Note that changing the permission of the list does not affect permission of the site.

 ---
 $site = new-object microsoft.sharepoint.spsite("http//server/site")
  $web = $site.rootweb
  $account = $web.ensureuser("user1")
  $web.roleassignments.remove($account)

$list = $web.Lists["lsit1"]
  $list.hasuniqueroleassignments
  True
  $list.roleassignments
 ---
  The permission right of user1 is deleted.

Execution environment
  sharepoint server 2007/2013
  windowsserver 2008 r2 64bit