I have 'Post Before Registering' enabled.

A guest posted, I can see the post with my admin account (I can see its Russian spam), however, I cannot access the content (I want to remove it).

 

I can see it:

 

But I cannot access it:

 

Anyone else?

Another one, how will I prevent this post from showing to users if this spammer responds to the Post before registration email and registers. 

 

This is a known issue that will be fixed (soon) in 4.4.2. The posts in the widget are however only visible with your admin account.

Glad to be a bug bounty hunter.

Where do I manage this?

The forum has been flooded with spammers since this was enabled (none fully registered yet (thank god)) how can I go in and delete these posts before they get the chance to register?

The content is NOT actually "posted". It is a bug that the post is showing in the widget, and this has been addressed in 4.4.2.

You cannot visit it (neither can anyone else) and it will be automatically pruned in a few days if the account does not complete their registration. In short, you should simply ignore those posts for now, and as mentioned 4.4.2 will stop displaying them in the sidebar blocks (which was the bug in this case).