I'll try to break this down into workable steps. I know how hard directions can be to follow.
1) Locate the "FAMILY" link on the menu bar located on the left-hand side of your browser.
2) Click it.
3) Locate the "Leader Options" link located on the right-hand side of your browser.
4) Click it.
4) Locate the "Declare" drop-down menus located below the "Manage players" heading.
5) In the first box, locate the rogue's empire name.
6) Click it.
7) In the second box, locate the word "Inactive."
8) Click it.
9) Locate the "Declare" button at the bottom of your browser.
10) Click it.
11) Remember the "FAMILY" link you located at the left side of your browser?
12) Click it.
13) Locate the coordinates link to your family's home system, it's just to the right of your family name.
14) Click it.
15) Locate the planet of the rogue empire. It will be the one with his name under it.
16) Click it.
17) Locate the "Attack!" link under the line which informs you of the ownership of that planet.
18) Click it.
19) Locate the "Use everything" link above the "Launch Attack!" button.
20) Click it.
21) Locate the "Launch Attack!" button below the "Use everything" link.
22) Click it.
23) Locate the "FLEET" link on the menu bar located on the left-hand side of your browser.
24) Click it.
25) Locate the fleet now waiting at the home planet of the rogue empire you wish to remove from your family.
26) Locate the attack link located within this fleet information.
27) Click it.
I'm just glad to help. Now you know how to remove a rogue empire and can RULE THE GALAXY! YEY!
[I wish I could obey forum rules]