i personnaly dont know, do you ¿

This is not an issue with the kata, your code is mutating the input. Try printing the original input right before you return to see what I mean.

I dont even understand the statement :)

If you could identify what was confusing, or have any suggestions how to clarify it, please share.

I wouldn't bother implementing a limit on a 7 kyu.

That's somewhat irrelevant because the same technique will work.

However, because this type of cheat takes a lot of chars (about 13K), you can prevent it completely by implementing a character limit to the submitted code size. Something like 500 would be fairly generous and still absolutely do the job.