How to remove an ADSL configuration?

[Thomas Dean]

For some reason, I used pppoeconf to establish an ADSL link to take
the place of eth0. But now, I'm going to delete the ADSL configuration
and use my eth0. .But I found that ifupdown in the Network Manager
couldn't be modified. After that I tried these:
1. Edit /etc/network/interfaces. I deleted all except lo.
2. Remove the directory /etc/ppp/
As a result, ifupdown was removed and eth0 came back, but eth0 became
unmodified.
I think the way above is wrong. So could you please tell me the right
way to delete the ADSL configuration?