How to deal with stupid clients who don't know about graphics and just ordering to make an impossible thing?

I don't think there are stupid clients, but they are not specialized in graphic design,  plus they are lost because they need the best for their company.

Actually client is the motivator of your analysis to create the best of designs and you both are working as a team to develop concepts and execution process... BUT

you have to set a strategy to deal with your client as well as he is doing the same:

first prepare your self with a deeply research of his brand and competitors

1- Gathering all requirements from clients in the first meetings, Don't tell IDEAs,

2- Second meeting give concepts and ideas and open a sharp discussion around objectives. ask a questions and prepare a check list.

3- back to develop your design well based on his objectives.

4- third meeting show the results, Don't do any changes front of him, or invite him to your studio, this is your kitchen and it is not allowed to invite people there.