Some companies block internet access except for websites that are used for work.
Be sure and lead by example, I work in a place that has blocked internet except for the leads. The leads continue to get on Social Networking Sites during work and have loud conversations about it, and call employees over to watch funny videos. It's kind of a double standard, and not really fair. Before our internet was blocked most employees didn't get on the internet for personal use until it was noticed that it seemed "acceptable" because of the use by superiors.
My other suggestion is keep the employee busy. If kept busy with projects, and sidework there isn't time for surfing the internet. (And if they are surfing the internet and not completing work, then they might need corrective action.)