Unfortunately there does not seem to be a function like ISALPHA or ISNUMERIC in SSIS. The only way I know of is to use a script component. See an example here
PS: My information is a little dated. I don't know if newer versions have this ability.
In standard SQL syntax, you would do this for "the last character" (as you stated). For multiple letters at the end, you'd need additional logic.
STUFF(column_name, LEN(column_name), CASE WHEN RIGHT(column_name, 1) LIKE '[A-Z]' THEN 1 ELSE 0 END, '')
For example:
select stuff(column_name, LEN(column_name), CASE WHEN RIGHT(column_name, 1) LIKE '[A-Z]' THEN 1 ELSE 0 END, '')
from (values('240261L'),('013022EG'),('18930')) as test_data(column_name)