Maybe something like this would help.

WITH SampleData(String) AS(

SELECT 'ADB' UNION ALL SELECT 'ADC. DCD.' UNION ALL SELECT 'ADC.'

)

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

THEN LEFT( String, LEN( String) - 1)

ELSE String END

FROM SampleData

Do you have more than one special character at the end? Could you give some sample data?