I know your question is where avast saves the blocked url so u can edit the file manually and add many urs in few buttons, Well I dont know BUT Since your objective is to just block the ads and get rid of the software doing it, You can use the way I do it.
By ediitng your host file http://www.mvps.org/winhelp2002/hosts.htm
This way
1) All ads sites will blocked at cost of no new software, No annoying banners
2) many many sites which spread viruses and spywares will be blocked
3) even if a spyware gets in ur pc, theres less chance it would be able to connect to its host and show banners/download malware etc