Deactivating or Deleting accounts on a web service

I am building a web service where the user registers and then can have access to this service.

During his/her time using the service, the user collects some points according to his/her actions (similar to what Stackoverflow does).

Question 1: Do we need to offer both DEACTIVATION and DELETE account?

Question 2: In case of DEACTIVATION should i offer an option to re-activate their previous account [with all points gathered so far]? Meaning all their previous data are kept in the database, just change some flags? Is there a best practice for this?

Question 3: In case of DELETE account, how do i proceed? Do i delete all database data regading that user? What if a user wants to delete his/her account because he/she gathered a lot of negative points and wants to re-register with the same email just with no negative points?

Question 4: Do other services (like Facebook or Google+) delete any data from their databases even if the user wants to delete his/her account? Are there any legal issues?

Thanks!