You should be able to expel a player from your pool that you are captain of by accessing the members management section:
1) Open your pool in the relevant tournament
2) Click on the cog icon to open the pool management section
3) Click on the Members button on the left to open the player management section
4) Hover your mouse pointer over the options icon (...) to the right of the player's name
5) Click on the Expel option on the dropdown list.
I hope that helps.