The ACE should uninstall when you uninstall the 757.

Try this:

Delete the 757 ACE shortcut in the Start Menu (in the Captain Sim folder).
Then go to the FSX\Captain_Sim\757 folder and delete the ace folder. If there is nothing else in the 757 folder, you can delete it as well. Doing that should get rid of everything for the 757 in the FSX\Captain_Sim folder.

Reinstall the 757 Captain, and it will install everything again.

Note. Reinstalling the 757 Captain normally overwrites the previous files anyway, so even if they are there, they are replaced when you reinstall.