Is there any pre-existing plugin to track and block IPs with suspicious activity on my site?

I use http://wordpress.org/extend/plugins/limit-login-attempts/ which blocks IPs when login attempts exceed set limit you set.

Limit Login Attempts blocks an Internet address from making further
attempts after a specified limit on retries is reached, making a
brute-force attack difficult or impossible.

If you’re on a host where you can install and run root code, look at http://sourceforge.net/projects/fail2ban/

“Fail2Ban monitors log files like /var/log/pwdfail or
/var/log/apache/error_log and bans failure-prone addresses. It updates
firewall rules to reject the IP address or executes user defined
commands.”