Here's a modification.

WITH SampleData(String) AS(

SELECT 'ADB' UNION ALL SELECT

'ADC. DCD.' UNION ALL SELECT

'ADC.' UNION ALL SELECT

'ABC DS.' UNION ALL SELECT

'AD.' UNION ALL SELECT

'DG@' UNION ALL SELECT

'DGC DS ' UNION ALL SELECT

'fdsf fs/' UNION ALL SELECT

'fs fds //'

)

SELECT CASE WHEN RIGHT( RTRIM(String), 1) LIKE '[^A-Za-z0-9]'

THEN LEFT( String, LEN( String) - PATINDEX('%[A-Za-z0-9]%', REVERSE(String)) + 1)

ELSE String END

FROM SampleData