They are trying to sell you a service, not do you a favor. Keep that in mind. I've had this experience several times (usually with financial services of various sorts, including insurance), and I've found that the best approach is to decline without explaining your concerns. If you tell them you haven't been working long enough to need their service, they'll tell you why everybody needs it. If you tell them you're concerned about giving them your tax number, they'll tell you why that's safe. If you tell them you're concerned about their bad reviews on Yelp, they'll tell you why that's all fake. And so on.
Instead, be brief, be firm, and leave no openings:
Thank you for your offer, but I am not interested.
That's it. If they push back, repeat it and add "please do not contact me again". If they keep pushing, hang up on them.
You said that they are affiliated with your accounting firm, and another answer suggests threatening to take your business elsewhere. I agree that if they are pushy or violate "don't contact me again" you should escalate, but don't do it to the person trying to sell you the service; he probably doesn't care. Instead, raise the matter with the people you already have the relationship with -- your accountant if you have a regular one or the manager otherwise. When escalating, unless you are prepared to take your business elsewhere right now, I've found that gradual escalation works best -- you don't want to make a threat you're not prepared to act on, but maybe you don't need to make a threat at all. Here are some ways that conversation might go:
You: I want to let you know about a negative experience I've had with one of your financial planners. (details)
Them: Thank you for letting us know. That is inappropriate and we will deal with it.
If it goes like that, you (probably) get a good outcome and you didn't have to put anything on the line. (Only time will tell if they actually took action. There's not much you can do about that.)
You: I want to let you know... (as above)
Them: Oh you can trust them. / We offer this service to all our customers. / That's just how they are. / (anything else that excuses their behavior or downplays your concern)
You: They wouldn't take 'no' for an answer. They continued to call after I told them to stop. This is inappropriate.
Them: (anything other than "we will deal with it")
You: I am reluctant to continue doing business with a company that treats me this way.
In this scenario you start by raising the issue (as in #1), then stress that it is a problem, and only if they continue to resist do you proceed to the soft threat. If even that doesn't get their attention, then you can begin the process of moving to another accounting firm on your schedule, rather than having to do it today because you just said you're firing them. (I'm assuming that you'd need some time to research and choose someone else and that there might be better and worse times to make a move. If none of that's true and you would be ready to walk out that day, you can be more direct.)