There is a suggestion ... Make module LIMITATION guests. The essence of the work: The guest can write a message, but only a limited number of characters. Let's say a guest can write no more than 20-30 characters. Number of characters can be set with the admin panel. This is a handy option in my opinion. Since cuts spammer attempts to add a huge number of options and other harmful information. Later, this module can further develop and implement a system of restrictions for those who do not obey the rules of the resource. Such a user also can set limits on the number of characters in the message.
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Ru
Есть такое предложение... Сделать модуль ОГРАНИЧЕНИЕ для гостей. Суть работы: Гость может писать сообщения, но только ограниченное количество символов. Скажем гостем можно написать не более 20-30 символов. Количество символов можно задавать с админ-панели. Это удобная опция на мой взгляд. Так как отсекает спамерские попытки добавления большого количества всяких ссылок и прочей неполезной информации. Позднее этот модуль можно доработать и внедрить систему ограничений для тех кто не соблюдает правила ресурса. Такому пользователю тоже можно поставить ограничения на количество символов в сообщении.

This is quite handy! Thanks!

Good idea. Would have realized this plugin.

The problem is, the word like 'fcuk you' only takes 8 chars and it's easy to overpass this for those who really want to do harm :

No, this is a word filter. The essence of another. Example:

http://blablabla.com/spam%spam%spam%/spamparam

Guest can add these options set.
A limit in characters truncates this possibility. Of course this is not a panacea for spam, but in other forums had ever seen.

It's not about limitation of characters at all, but about limitation for guests to not post an URL or web address. And - we already have such module here:

" Anti guest addon " - Reducing spam - guests cannot post links

Limitation of characters is really annoying in terms of posting for guests. Guests may also post a very useful message. Here on our board we have at least 15% of useful and stored messages posted by guests. I doubt that miniBB would be useful if there would be such "limitations" introduced. I am personally against such thing, it's more seems like a limitation of free thinking than preventing spam.

I am agree with Paul : if you have allowed guests to post, so let them talk ! ;)

What can I say? And I agree ... However Baule that you want an addon (or rather alternative) is.