I assume they have been given an assured shorthold tenancy, as that is the normal type. If so, you cannot get them out against their will without a Court order, which will take time. You need to make sure that they are breaching the tenancy terms set out in the contract. (If there is any doubt about that, it means the contract is not worded properly - which presumably is the fault of your agent.)
If they are breaching the terms it is no good the agent just talking to them. They must be given a letter setting out the breaches & the consequences if there are any further ones. This should either be delivered by hand in the presence of an independent person who can confirm in Court if needed that they have received it, or be sent by recorded delivery. A copy must be kept.
The letter will provide evidence if it is then necessary to go to Court - i.e. if the breaches continue.
It would also be a good idea to get written complaints from the other tenants detailing the grounds for complaint, as these also coul;d be needed for Court evidence.
Once this is all over sack the agent & get one you can rely on, or do it yourself. Don't tell the agent now that you will be doing this, as they would then have no incentive to sort things out.
Another thing the other tenants can do is to involve the local authority environmental health team, both about any health issues & about undue noise at anti-social hours.