Delete statement not responding to Kill session command

Four days ago a user ran the command below on a table with 400,000,000 rows. It's still running and the log file is increasing in size. delete from [table-name] This table does have a foreign key constraint which is not enabled for checking and I know that no rows exist in the other table. The database is running with "Is Read Committed Snapshot On" enabled and in Simple recovery mode. After this had run for a few hours I issued a kill session command, because we were running out of disk space for the log file. I added another log file to allow the system to continue to function. The log file is continuing to grow and when I run kill session with statusonly it returns this message: SPID 123: transaction rollback in progress. Estimated rollback completion: 0%. Estimated time remaining: 0 seconds. I am at a loss as to what to do about this query to get it to rollback and also just understanding what is going on, can anyone suggest what I can look at?