Many thanks for your patience, I am pleased to be able to continue assisting with your query now. First of all, I am sorry to hear about this situation and any associated issues.
Because the bullying is coming from a third party, you cannot force them to do anything to deal with this and your best option would be to go through your employer.
However, they also cannot force that agent to do anything about it and all they can realistically do is highlight this and raise their concerns and disappointment that it is happening. Of course, they can say that they will cut their ties with them if this continues, but that is not something they have to do and they have to consider what is more important to them – retaining you or their partnership with that agent.
In the end, if you are bullied and the employer is not doing anything to try and deal with that, your only option would be to resign and claim constructive dismissal. There is, however, a requirement for the victim to have at least 2 years’ continuous service with that employer (which does actually mean that those with less than 2 years’ service cannot make a claim and can effectively be bullied out with no recourse). However, the 2-year limit is not required if the grounds for bulling were discriminatory, such as because of gender, age, race, religion, disability, sexual orientation, etc. I can see no evidence that discrimination is at play here, which means that you could be left with the option of either staying there and maki ng the best of the situation, or leaving, without the ability to make any sort of claim about it.