RouterOS v6.27
How can I block wireless clients from joining my wireless network?
Can I set static IP addresses for known clients, and only allow these known clients to access my network?
How can I block P2P network traffic?
Add client MAC addresses to the access list which you want dropped.
Unless they’re clever enough to spoof MAC address, they’ll be denied.
(next step = change the password)
(next step = require certificates, make one for your laptop and then require exactly that cert - this is the most extreme)
(you ARE already using aes/sha1 encryption right?)
If not - stop reading this and go set it up NOW!
For “deny all except:” policy…
Here’s a howto with pictures:
http://computechtips.com/7/mac-address-filtering-mikrotik-wireless
(it was actually not my first hit on Google, amazingly)
There are some p2p blocking threads on this forum already.
Search for them - the best solutions today block using a content matcher that looks for tracker announce information in udp payload.
If they’re your kids, then I suggest “if I catch you doing it, then there is going to be consequences” type of “layer 8” firewall. 
If you suspect they’re doing bittorrent, then go into firewall > connections and if you see a gajillion connections from their IP to a gajillion random looking public IPs all on the same weird port, then they’re doing it. Ask “who’s doing it?” and if the traffic goes away and you hear “NOBODY!” - then you know the MAC and who did it anyway - make them paranoid about what you can see.
ZeroByte. Thank you for replying back.
The website you provided helped me. Funny enough, I also Google’d and found the same resource.
However, I’m having a tough time figuring out how to block P2P type of traffic. I’m searching the forum, but I’m not finding something definitely helpful. Can you provide me a link, or instructions, on your proposed solution (i.e., block content matches/tracker announced information in UDP payload)?