At least 70,000 "graphic" images were found on one computer
Lawrence Bailey, 52, was found to have breached codes of conduct by the Adjudication Panel for Wales.
It also found he had broken rules by writing to a local newspaper using false names.
Last year Mr Bailey was cleared of any criminal activity over the pictures found on council-issued computers.
A former Lord Mayor of the city, he was arrested after his computer was sent to the council's repairs department.
He faced a three-month investigation before being told by the Crown Prosecution Service that no proceedings would be pursued.
He resigned as a councillor on Swansea Council in May 2007.
The adjudication panel's report found that three separate computers used by Mr Bailey held indecent images.
Council engineers discovered 6,000 adult images on one laptop's hard-drive.
In some cases the addresses did not exist or the postcodes did not match them, or the true residents had no knowledge of the letters.
Adjudication panel findings on letters to the press
It was replaced, but in 2005 several thousand more pornographic items were discovered on the same machine.
Another computer used by Mr Bailey was discovered to hold at least 70,000 graphic images, of which half were described as "adult hard-core pornography".
The third computer, also a laptop machine, also held indecent pictures.
But the panel also found it contained 253 documents, mostly letters to the Swansea Evening Post.
"Although the police examination showed Mr Bailey as being the author, the documents purported to have originated from various residents of Swansea," stated the report.
"In some cases the addresses did not exist or the postcodes did not match them, or the true residents had no knowledge of the letters."
In his own submission to the panel, Mr Bailey claimed that writing letters to the press under pseudonyms "was widespread practice in political life in Wales".
Publishing its decision, the adjudication panel accepted that Mr Bailey himself admitted misusing computer equipment supplied by the council, and had brought the office of councillor into disrepute.
But banning him from standing as a councillor anywhere in Wales for two-and-a-half years, the panel noted a series of what it described as "aggravating features".
These included the persistent downloading of images over five years, failure to heed advice given on two occasions about his conduct, and the "serious undermining of public confidence" by his actions.Swansea Council has declined to comment on the panel's decision.