Welcome to Android Forums! On the Galaxy Ace device, open contacts (or dialer and go to contacts), press on the contact name, the menu button, and select delete contact.
If you need to delete multiple contacts, press the menu button while viewing all contacts and select Delete contacts, and this will allow you to select multiple contacts for deletion.
In addition if you are using Google to sync your account you can log into your gmail account, select contacts and remove them from there by clicking the orange "GMAIL" right below the Google icon, and selecting contacts. You can then check the contacts you want to remove and click on the more button listed above the contacts and select delete.
I hope this was helpful! If you have any further questions please feel to post another question in the same manner. We enjoy helping visitors, and would love for you to join our site as a member so you can interact with the entire community!