Bots

[Ventic]

any way to avoid bots?

[Shambles]

What's your understanding of a "bot"?

[Ventic]

they register and making advertisments topics

[MovedGoalPosts]

Catchpas are often read by bots, when humans can't read them so don't rely on them alone.  Indeed I have turned them off completely on my forum.

I use the verification questions, a random choice of say 2 out of 10.  But that may not be an option if your forum has multiple members.

Have a look for the various anti spam type mods.  Anti-Spam

[Shambles]

What's your understanding of a "bot"?

they register and making advertisments topics

What about googlebot, baidubot and msnbot? They're also bots but they don't tend to register accounts or do anything nefarious, and you wouldn't really want to purposely "avoid them" 

So, what you need is anti-spambot software for which movedgoalposts gave a good link