Hi @BitSeeker. There isn't an official rule against it. However, general abuse of the message system is against the rules. For example, it would not be responsible for someone to send unsolicited requests for assistance to many forum members. If you believe someone is abusing the message system, you are welcome to flag the messages.
I respond with a stock message:
We prefer to help people publicly here on the forum because this way the information shared is available to everyone.
There are millions of Arduino users and we don't have time to help each one individually. The hope is that when I help someone in a public forum topic, that information will help many others with the same question in the years to come as they find that forum topic in search results. In this way, the effort of one person is magnified many times through the magic of sharing information freely on the Internet.
If you want help, make a public post on the forum like everyone else does.