For those who are curious on why Dean hasn't done this already: This is a limitation of XenForo's default settings. He could change this in two ways - a plugin or by programming it himself. I don't think Dean has much experience with programming himself (I could be wrong), so we would have to assume by default he would need to get a plugin for this. The problem with this, this normally costs money.
The most popular plugin of this nature is AddonFlare's
XF2 Ignore/Block Essentials plugin.

I mean that's kind of brutal for a true block feature, I imagine.
Ignoring the price of it, in general plugins can be kind of a pain in the ass. They are not something easy to manage if you're not a programmer. They interact with the site's 'core', and so that means a plugin can break the rest of the site. Troubleshooting will be difficult if you don't know to troubleshoot, and XenForo will not provide support for errors caused by plugins.
Some security factors to consider is: 1. it's hard to verify how much information third party plugins may collect on the site, and counting the site is one of the more popular ones, 2. it might make the site more vulnerable to being breached. This is not normally a concern to the average website using XenForo and their plugin feature, but still something to keep in mind.
It's about weighing the pros and cons in this case. While I understand why this is a very popular feature to request, I also understand why Dean might be hesitant to move forward with it. There's not many benefits to adding it, but there's potential pains it may cause. If you are not a programmer, it's always better to have less plugins installed whenever possible.
(For
@Dean, if he decides to read this thread):
I found an alternative by NeyCruzz, but I'm not able to verify the files or anything because I don't purchase the XenForo suite. If it's a free plugin, you can try sending me the files, and I can a very minor glance at them. If it's not too big, I can take a deeper look at them and help you install it.