Everyone knows and Riot does know about it aswell. They're in the development of means to prevent this. You can read Riot tmx's post here

we're obviously aware of the friend request spam problem as we're also getting tons of requests every day. I synced up with our Game Designers and discussed various solutions that could be put in place. We have some ideas that are currently in development, but we also know that spammers regularly adapt their strategy to overcome constraints we put in their way. This is not the first time we defend our players from the spamming problem, so we're approaching it a bit differently.

Anyway, we're going to implement some mechanics to stop/reduce the amount of requests, but additionally, we're working on solutions that will let you gather more information about the person that is inviting you. Things like a link to their profile, their level, etc, should help you to distuinguish a legit player from a bot. I'm planning on giving you some more details soon, or maybe ask our Game Designers to describe the problem and our plans form their point of view.

For now, don't accept them and don't deny them. It'll calm down. If you really want to not receive bot invites, LSI provides a solution by allowing you to automatically deny any level 1 friend invites. That should do the trick until Riot implements their systems to counter it.

Here's a link to League LSI, it's completely fine to use as it is and you won't get banned for it.

This has been repeatedly posted those last few days and I can assure you that Riot knows the priority this takes, please do a quick search next time.