How to kill or rollback active transaction?

You can't kill/rollback a transaction from another session without killing the owner session.

I think, allowing to kill/rollback a transaction from another user's session means many design and security rule violations because it requires entering another user session (in the context of the current sql server engine design). That's probably why it is not implemented.

To clear all transaction, Use this only as a temp solution

ALTER DATABASE DatabaseName

SET SINGLE_USER

WITH ROLLBACK IMMEDIATE;

ALTER DATABASE DatabaseName

SET MULTI_USER;

refer this site for more info

Points to check - it also rollbacks committed transactions