In the SQL Server Management Studio, to find out details of the active transaction, execute following command
DBCC opentran()
You will get the detail of the active transaction, then from the SPID of the active transaction, get the detail about the SPID using following commands
exec sp_who2 <SPID>
exec sp_lock <SPID>
For example, if SPID is 69 then execute the command as
exec sp_who2 69
exec sp_lock 69
Now , you can kill that process using the following command
KILL 69
I hope this helps :)