Is there any way to only show what is said on the window without showing everytime someone logs in or out (filling up the screen)?
Right click (or left click, i'm running mIRC in wine and right click does nothing for me) on the left icon in your chat window title bar to bring up a menu. Select events, and you can then choose which events to display in that particular window.
Alternatively, you can go to Tools->Options->IRC. From there you can set these same event options as default.