The game retailer calls Riot's decision "heavy handed and potentially damaging," and "an aggressive attack directed towards the entire global esports community in a failed attempt to dominate and control the industry."
Last week, Riot ordered teams sponsored by G2A to remove logos from their merchandise at the World Championships.
G2A has spent $6 million on eSports sponsorships over the past two years.
The logo ban came as a result of disagreements surrounding G2A's sales of pre-leveled LoL accounts and Elo boosts.
According to G2A, the ban came after many concessions had been made, but "there were only more and more demands from Riot."
"Riot even asked G2A to remove helpful tools for players such as game guides," continues the press release.
G2A is seeking feedback from eSports fans about how they're affected by the ban, but Riot maintains that the decision is "final."
The League of Legends World Championships are in the group stage right now.