I also experienced this issue just now. The database and reporting services instances are on 10.50.2500. I killed the transactions as per DBCC OPENTRAN (I didn't want to take the whole instance down). Reporting Services didn't stop after and users were able to run reports. I'm not saying this will work for everyone but it worked for me. In hindsight I should have used fn_dblog to track down what was actually filling the log.