В вашем application.conf
установите akka.http.server.server-header
на пустую строку:
akka.http.server.server-header = ""
Из эталонной конфигурации :
akka.http {
server {
# The default value of the `Server` header to produce if no
# explicit `Server`-header was included in a response.
# If this value is the empty string and no header was included in
# the request, no `Server` header will be rendered at all.
server-header = akka-http/${akka.http.version}
...
}
...
}