Bak is not a brk, but a backup file of cad. It is recommended to keep it, because when your source file is lost, you can change the suffix of the backup file to dwg format and you can use it. If you want to delete bak, the method is Tools-Options-Open and Save, find the column of file security measures, and find the check in front of it in the column just deleted.

As for lisp, it should be cad poisoning. You can find a killer online.