Suppose that...

[Nicholas the Italian]

...you have a user who keeps annoying and provoking other people in your board even after repeated warnings, so you're willing to ban him; but you fear that he will try to get back in the forum in some way, writing as a guest (since this is allowed) or re-subscribing with another identity.
What countermeasures do you take? What are the chances that he'll be able to actually keep disturbing?
Is IP-banning effective? What's the risk to cut out other people?
What else would you do?

[camm15h]

Administrative account activation.

[Nicholas the Italian]

Administrative account activation.

Which is a thing I'd like to avoid... anyway, good one.
So, I must presume there's no way to keep someone out, if I want guests to be able to post in certain places?

[Uchiha Nick]

Administrative account activation.

so.. if he tries to come through another username, and fills in false data?

if someone REALLY wants to mess with your board, you practically can't stop him. you can thank dynamic IP's and 5min email creation forms

[Nicholas the Italian]

if someone REALLY wants to mess with your board, you practically can't stop him. you can thank dynamic IP's and 5min email creation forms

I thought so.
So, a ban is basically like saying "sorry you're not welcome here, we ask you not to come back"...