Hi
First of all , you need to check whether your company has a internet usage policy, if you have then you can circulate an warning letter for using job portal sites during usage. You can direct mention the policy and send out warning letter as inappropriate use of internet during working hours. If you dont have the policy, then you have to create a policy, circulate to all employee's and then if any found, you can issue a warning letter.
In my company, all social sites are blocked including FB. So better you can ask your IT person to block all irrelevant website which will help you on a larger scale.