Code inserted from CP POLLS after voting

We have notices since the last months that some kind of strange MYSQL code was inserted in our CP Polls – Report/Results. Although we have limited and blocked the votes at 1 per IP, some malicious user vote more than 500 times in a couple of hours.

We want to know if there is something we can do to make the polls safer from this attacks.

We are working with WordPress 4.1.1 and CP POLLS 1.0.17 .

Our website is https://www.anticonceptivoshoy.com/