The ads for Natwest and RBS both included scenes where bank employees pledged their banks would "continue to provide banking services wherever we are the last bank in town".
However, a town councillor and the Campaign for Commuity Banking Services complained there are some instances in which the banks have withdrawn banking services where there are no other branches.
The complainants said Natwest, which is owned by RBS Group, had closed a branch in Farsley, Yorkshire despite being the last bank open in the area.
RBS had substantially reduced its opening hours or replaced branches with a visit from a mobile bank in some locations, the complainants added.
RBS said the ads did not specify that the bank would keep branches open, but that it would only continue to provide "banking services" to certain areas.
The bank argued it is keeping to this pledge by offering services through branches within a larger area, or via mobile banks.
However, the ASA ruled the claims made in RBS and Natwest's ads were likely to be understood by consumers that there would not be bank closures or a reduction in opening hours, and therefore were misleading.