Those are humans, they can come up with new ways to fool the filter rather easily...
Yes, excactly. Another example:

Posts a thread, notices it gets auto-censored. Post it again in a way that bypassing censorship.
What is the point of using the unicode characters to bypass the filter? I assumed that they spam forums to boost their relevance as seen by search engines. Search engines would not pick up on misspellings so their aim must be to advertise to users of the forum.
Search engines might be "clever" enough to figure out miss-spelled words.
Or at least the spammers
think that search engines work that way.
I somehow doubt the purpose is advertising to users directly, the messages are too stupid and sometimes are just bunch of keywords.
I wonder if they are silently reading that thread to get info about the filters set against them.
Not sure, but I think it would not even be nessecary. It does not need any "inside-info" to come up with something like above. And any other plugin or other solution we might try, other forums have likely tried before us. So they have likely seen all possible obstacles beforehand anyway.
Either force people to respond to questions.
They are humans.
Spammers can answer questions like "What color is the sky? Answer:blue" just as well as normal users.
There would be the option to have spring-specifique questions but they must also be simple enough that new users know the answer, too.
I think no such question exists.
Even if such question existed: They could just spend very little time to figure out the answer, then store it in some database for future registrations.
Manually approving users:
-How do you know, before he posted, that he will not post spam?
-risks turning away valid users, if they have to wait.