Check if a variable contains any non-numeric digits in SQL Server
Check for any characters that are not in the range 0 to 9
IF @rptID LIKE '%[^0-9]%' --throw error
There is also an ISNUMERIC function in MSSQL if you're using version 2008 or later.
IF (not ISNUMERIC(@rptID) = 1) THEN throw Error