Deleting non-active users (e.g., users who didn't log in for 90 days)
Maybe this module can help: inactive user

From the module page:


  After configurable periods of inactivity, one or more of the following
  actions can be automatically taken:
  
  
  send an email to the user  
  send an email to the site administrator(s)
  block the account (a warning can first be issued)
  optionally notify user and/or site administrator(s) that account
  was blocked  
  delete the account (a warning can first be issued) 
  optionally notify user and/or site administrator(s)1 that account was
  deleted