Use the annotation org.freedesktop.DBus.Method.NoReply"
Example XML:
<node>
<interface name="org.test.example">
<method name="NoReplyMethod">
<arg name="data" direction="in" type="i"/>
<annotation name="org.freedesktop.DBus.Method.NoReply" value="true"/>
</method>
</interface>
</node>
Generated Code:
void NoReplyMethod(const int32_t& data)
{
::DBus::CallMessage call;
::DBus::MessageIter wi = call.writer();
wi << data;
call.member("NoReplyMethod");
assert (invoke_method_noreply (call));
}