depending on what access you have to the SQL instance you may or may not be able to do this

Run SQL Server Management Studio and check to see if the ReportServerTempDB is in Simple recovery mode (right click properties, options), if its not set it to simple recovery. This won't recover the disk space but will clear the transaction which i'm presuming is not getting backed up. To get the space back run this in a new query, replace the 1000 to whatever you want the new size to be in MB.

USE [ReportServerTempDB]

GO

DBCC SHRINKFILE ('ReportServerTempDB_log' , 1000)

GO

you can set the DB to simple recovery using this as well

USE MASTER

GO

ALTER DATABASE [ReportServerTempDB] SET RECOVERY SIMPLE

GO