Rees was subsequently suspended up until March 31 but there was talk that the Blues and the WRU would contest that decision in the hope that the verdict would be overturned or the length of the ban reduced.
However, after receiving the written verdict of the decision, club and country have decided not to do so.
"Having sought legal advice, with the support of the Blues and WRU, we have decided not to appeal against the decision," said Rees, who was expected to be in the Wales 22 to face England in the Six Nations opener on Friday, February 4.
"Naturally I am disappointed at the outcome of the hearing and to be suspended for 12 weeks. Missing out on the Six Nations and some crucial games for the Cardiff Blues is hugely disappointing and hard to accept.
"I felt it was worth looking at the possibility of an appeal as I am desperate to play for the Blues and Wales and naturally feel frustrated and disappointed at the outcome.
"However, I will now be concentrating on training hard in anticipation of the Magners League run in for the Blues in March and April and hopefully can still be part of the Rugby World Cup preparations with Wales."
As well as the forthcoming Six Nations, Rees will be unavailable for selection for the Blues' two remaining Heineken Cup fixtures and five Magners League fixtures.