What version of SQL Server are you running? Newer versions of SQL have better string support but ultimately this is still going to be painful. It is also highly dependent upon how reliable your strings are going to be. Here's something that would work in SQL 2016 and above.
DECLARE @tests TABLE (value VARCHAR(100))
INSERT INTO @tests (value) VALUES
('Bob, Smith A ET UX'),
('June, Smith A ET AL'),
('Mark, Smith A ETAL AL'),
('Susan, Smith A ETUX')
SELECT *, TRIM(SUBSTRING(value, CHARINDEX(' ET', value, 0), 1000)) FROM @tests