The best thing I thinki you can do is spray them with water everytime they climb on the chairs, when you spray them, it is worth shouting "NO!" as you do it. They will not like the shouting or the spray and with a bit of persistance will get the message that these chairs are not for them.

When you are out of the house, either leave stuff stacked on the chairs to keep them off, or shut them out of that room, that way they will not try to sit on them when you are not alone.

It shouldn't take too long for them to lern that they are people seats and not cat seats. Best to start with this sort of training young, and keep it up, otherwise you will just confuse them.