I would like to ask you to do two things. First, pretend that this is a forums rules or other required reading. Second, pretend that you are one of the idiots that are trying to sign up without reading anything.
I have both moderated and contributed to many forums over the years. Like anywhere online where anyone can post, you run into a big problem with forums: inconsiderate selfish idiots. People who have no respect for anyone else and who expect everything to be done for them. These are the people that don't read the rules and don't use the search button. They simply post and expect people to help them no matter how many times their question has been answered or no matter how inappropriate it is for the particular forum.
I have seen many approaches in my day to deal with those type of people. One method is to have a minimum post count that restricts users to certain forums based on how many posts they have.
Another approach is to make users read through flagged posts and explain what that person did wrong and how they could have avoided that.
Unfortunately both of these approaches have problems. The first one leads to tons of useless posts. I do mean useless. I've seen people post "post one", "post two"... until they've reached the requirement.
The second method requires the moderators to keep old/bad threads for people to look through, which is more work than simply deleting them. It also isn't automated. For large scale forums it isn't practical to have people PM'ing a moderator to get their privileges escalated.
While my solution doesn't keep out the idiots and jerks and it doesn't make people post well, it does ensure that people actually read whatever information you require them to.
This page is a working example of my suggestion. The idea is to embed a hidden link that will grant whoever presses it access to a controlled place. By 'hidden' I mean the link isn't obvious because css has been employed to make it look like regular text. It is also 'hidden' among other bogus links that will flag the user as a possible trouble person. The idea is to tell whoever is actually reading this to click on the appropriate link instead of any of the others.
For instance, I could tell you to depress your mouse over the word deal in the third paragraph and only that. It would also be important to tell you not to follow the instructions on the last line of the last paragraph.
One thing that every forum will get to decide is what to do with people who don't read the rules. You could simply block them as soon as they click one of the wrong links. You could flag them so you know to keep an eye out. You could even have them run through a second test in case the first one was just an accident.
Another nice effect that this approach has is that it keeps out robots. Yes, a robot could be programmed to read the document and look for the secret link, but it's at least a good line of defense. If you're actually reading this, click the fifth word in the second paragraph.