This is really the perfect question to ask your Realtor. "What do I do, if I want to attend an Open House, without you?".
Typically, Open Houses are "no man's land". You are free to view an open house, without worrying that the agent sitting the Open will automatically become your agent for the purchase of that house. But Open Houses are a tool that agents use to try to attract new clients, so the agent sitting the Open will likely try to "schmooze" you (a real-estate technical term)... and get you to like them and then use them to help you buy a house.
Your agent will probably tell you that when you attend an Open to tell the sitting agent that you "are currently working with an agent", and if/when you sign-in on the attendance sheet, to sign in with his/her name next to yours. If you do so, you probably won't be bothered by follow-up calls after the open house.
Good luck, and happy hunting.