Ruined and Unacceptable are both interesting word choices.
Ruined would indicate that those 150+ games held little-no value for the other 4*150= 600 participants. On the contrary, I think that botted accounts improve the 150+ games' usefulness. Many games you will be matched with a tilted or trolling player that puts your team behind. Learning to keep calm and persevere in making what impact you can for your team is a very important lesson.
The best way for any individual to improve their skills in any setting is to engage in activities that are challenging to their current skill level. Having 4 botted accounts in a game is pretty close to playing PvP, and as such it is a useful tool for improving.
Unacceptable would indicate that there are other options available that are acceptable.
Banning more quickly would create even more bots, because the bot makers would need more bots active at any one time to meet the demand from purchasers*. Banning more often would also increase iteration. Then we'd eventually end up with bot accounts that are impossible for Riot to catch at all.
Unless you have a different solution in mind, "Unacceptable" seems to be the wrong word to use.
(*see any research that tracks birthrate and survival rate correlation)