How to Block all traffic that doesn't start with certain string question

does the case matter on that path? what if it comes in /chd, is that still ok?

This should work (assumes pool pool_soa_wservicetst_80 is the default pool):

when HTTP_REQUEST {
  if { not([string tolower [HTTP::path]] starts_with "/chd") } { HTTP::respond 403 content "Blocked!" }
}