That is a good practice, Eduardo.
In my forum we have an area where guest can advertise, funny enough it makes easier for me to decide which accounts to delete. It is stated clearly that if you post outside this area the thread gets deleted. And if you registered and just posted an advertise, your account will be gone.
I also took a tip from someone else, about censoring the sites they announce, and now I do that as well.
Now, about people that make spammy forums, I am... kind of flexible, depending the area, I usually first try to inform them and give them tips to enhanced their posts. Depending their attitude it can be forgotten or some serious measure added. I usually just suspend them for 2 weeks, if they do more then a month, then 2 months, then 6 months, then a year... and so on. If I catch a multiple account to evade the ban, I delete it in sign and double the suspension time he or she had left for each account.
If someone made the extra account some months after and pretended to be someone else, so long it respects the rules and stuff, I don't really going to ban him, or even make her/him aware we know who they are.
Now I haven't had yet to go this far with anyone.