pptp/vpn server under attack

I have configured pptp server on my mikrotik so that I can can connect from outdoor locations for some lan management and other purposes. (at home i have different isps with dynamic ips)

Problem is that I see a lot of attempts from so many public IP addresses trying to connect with pptp server, How can I block any ip address with 3 or mroe login attempts? or in short, how can I secure my pptp server?

You may try to adopt second method:
http://wiki.mikrotik.com/wiki/Bruteforce_login_prevention