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