I agree with Sindy, PPTP should not be used on the public internet.
What you can do.
- Block the 92.63.194.0/24 net.
- Use a script that block the ip if wrong username is used.
One of these scripts should work with just some small modification:
http://forum.mikrotik.com/t/black-list-for-failed-login-to-ipsec-vpn/130090/2
http://forum.mikrotik.com/t/can-a-script-be-created-if-a-wrong-login-name-is-used/132192/6
PS I do use some filter rules, that block the IP for 24 hours if some tries to access one port that its not open.
This way they get blocked before they find open ports and start to script attack those ports.