Code:
replace(replace(myfield,"S - Bayside Peninsula -",""),"S - Bayside Peninsula",""))
The idea is, that 1st replace searches for longer string, and when it finds it, then replaces it with empty string, otherwise doesn't change it. After that, second replace searches for shorter string (when 1st search did replacing, then obviously it is not found now), and when it finds it, then replaces it with empty string, otherwise doesn't change it.
(Ajax solution works only, when dash is not used anywhere else in same field in any row. Probably it will work too, but it is not foolproof.)