Don't refuse to see her - it will set alarm bells ringing and draw unwanted attention, especially if she has any concerns about how you are coping. How old is your baby and does the HV actually have any reasons to be particularly concerned about you? Are you a lone parent with a history of depression for example?
Stay calm and practise assertiveness. Look her in the eye, smile sweetly and say 'Look. I appreciate that you are paid to tell all new mums that breast is best, and I appreciate that you may passionately agree with that. I have made a decision that my child will now be moved onto formula milk. It is my choice, I have a right to that choice, and I believe it is the right thing for me and my baby at this time. It is not up for discussion. Thank you for your concern and your advice and please do not be offended or alarmed if I choose not to take it.'
Take the details of the groups she is pushing on you and say 'ok thanks, I might check them out if I am at a loose end or feeling bored/lonely but at the moment I have plenty of support and company from friends and I'm doing fine.'
If she constantly phones you and hassles you to go, then say to her 'Is it the law? Am I obligated to go? Will my child be removed by social services if I don't go? Because forgive me, but you are starting to make it sound as though it's not actually my decision on how I spend my free time. Are you on commission from these places or something?'
But always stay calm, always keep a civil tongue in your head, always remember that she is not your enemy (unless you give her good reason to be) and she is trying to help. She has your and your DD's best interests at heart, whether you think you recognise that you need that help or not.