Intro

One of the most misunderstood parts of leadership is how to deal with hostility. Yet, this is one of the things that will tell a good leader from an average one.
First of all, we all want to look as professional as possible, right? It doesn’t exclude fun, it just attracts more members. Quoting the example found here,

“Hello, would you by any chance be interested in joining a guild I’m starting up? It’s going to be PvP based. A vent server is already set up. I’m looking for a few quality warriors such as yourself if you’re interested…”

will attract more members than for example

“hi u wan 2 join me pvp guild? Lookin 4 warz like u, hav vent servr, msg me k”

Scenario

Let us imagine you dislike a clan (or its leader) for some reason. Perhaps they keep crushing you in any fight you have? Maybe their leader is too full of himself? Possibly, they even tried to destroy your clan?

What do you think is going to look more professional? Saying “Oh my god, I hate you guys, I’m going to kill your clan and hack your website!!”, or keeping quiet? Vice-versa, what is going to sound more professional here, in response to this?
“You’re going to hack us? Well, we’ll hack you first! Your clan is going to die!”

Or

“You’re going to try and kill our clan because you dislike us, in a game? Good luck with that chap, I won’t lower myself to that level.”

Exactly!
Whilst a cheeky, defiant response will inflate your ego a lot more, it will also make you look rather silly. The thing to remember is that it’s a game, and shouting “ur clan dies nao!!” just won’t help anyone. If you need to blow off some steam, do so in private with someone you trust to keep quiet, but don’t claim you’re going to “kill” a clan.

Patience

As said, trying to kill another clan is, in short, immature, when it’s a game you’re playing. Lowering yourself to the level of someone who threatens to kill your clan is no different. Though some people will quite like this, you will be scaring away a lot of potential members. Scrims, PVP, and the like, should always be done for fun.
The best way to handle an annoying overzealous clanleader, or a plain troublemaker, is staying calm and trying to avoid a conflict.

Measures

If someone is trying to kill your clan, there are a few measures you can take.

1. Always have your website and forum software, as well as other web applications, up to date. Additionally, allowing full HTML in, for instance, posts on a forum, is a formula for failure. This is one of the most exploited features of Invisionfree software. The same goes for flash content anyone can post.
2. If you are running your own server, always use a mod with security enhancements if the game you are playing is known to have exploits. You might also consider asking your server host if there are any security enhancements they can install. This difers per game though, and is mostly seen in FPS games.
3. Keep your calm. All the other clan will do is embarrass themselves, really. Do not ban them from your forums (and potentially server) unless they actually did something in violation of your rules.
4. If members of another clan are actively harassing your members, have hard evidence and report it to whoever is capable of ridding you of them. In MMORPGS, this will be a server administrator, in FPS games, this will usually be yourself.

Final thoughts

Finally, whatever you do, please remember that it is a game that you and your clan are playing. You are doing it for your amusement, not to harass others. The more professionally you present your clan in conflicts, the more members it will eventually gain.