how can i make a party member leave? because i met madras in trynton and i cannot let him join without removing a party member. how can i do that?

Right click on the character to get to the main screen for the character. On the picture of the character (Top Left) there should be an arrow, click on this arrow to dismiss character.

You can also dismiss a dead member. But, if you
do, this member is gone forever from the game.