It would be very cool if Crytek made some way to register a clan, so random players can't use clan-tag without actually being official member of that clan!

I'm also so tired of cheaters using clantags of some popular clans, just to ruin their reputation.
Please help us with this Crytek!

Agreed. This is a most pressing concern!

However, this matter needs some consideration, as if you make clantags short, there's only so many combinations to go around, and if you make them long, you can clog up space avobe people's heads...

How about the following: The clan tags can be reused indefinitely by any amount of clans, but when you bring up the scoreboard (TAB key, I believe it's select in consoles? I dunno) it tells you the clan the tag belongs to.

Clans themselves have to be registered in-game, will have a leader and chosen "admins" (chosen by the leader that is). The clan will have a clan name (which can be long but must be unique) and a clan tag (which may be short and reused).

That way you can keep short tags (three-five letters) and avoid facing the problem of too many clans for the amount of available tags!

I liked the way Quake Live manages clans, but the only downside in QL is the fact you have to buy account to register clan. But everything else related to registering clans in Quake Live is awesome. Your clan-tag can be shown only when you get in that clan, and to join the clan you need to be invited.
No false fake clan-tags, no using clan-tags by wannabes, no ruining anyones clan reputation...

Warface has it so you spend hard earned in-game cash to create a clan and then you invite players to join. We've already heard some features of Warface MP are being used in Crysis 3, so chances are we could see this implemented too. Would be pretty cool.

I'm also so tired of cheaters using clantags of some popular clans, just to ruin their reputation.

I can′t count how many times my name and tag in another game have been stolen.... Such an annoyance, but funny that those who did it couldn′t even get the 10% of my kills amount.