I cannot speak to other countries, but I can speak to U.S. law. Bare with me while I explain a few things.
If it is a factual post or primarily opinion, you are stuck. Nothing you can do about truthful statements or opinions. After all, there is the 1st. amendment.
However, if the post is false, based upon a faulty premise, or an opinion that is inflammatory, then you may be able to make a defamation or slander claim. Defamation speaks to reputation only where as slander is intentional falsehood designed to cause damage. You would have to know which one you are dealing with.
With defamation, person to person, then you have no real or little case. It is opinion. However, defamation person to entity or entity to entity, then you have a grievance.
With slander, person to person, person to entity, or entity to entity, then you have a grievance.
What is required to take action?
Proof of damages, proof of falsehood, and intent. Slander requires intent. Defamation does not.
It is possible to have a grievance if a person/entity makes a statement that is untrue with gross negligence or a person/entity makes a statement that is knowingly false at the time the statement was made.
Cease and desist letter: You can hire an attorney to send and file with the courts a cease and desist letter that provides the opportunity to make right the situation, providing that the false statement be removed within a certain period of time usually 10 days (not business days) from the date of the letter.
File a civil claim: After the cease and desist letter time limit having expired, you will be free to file a civil law suit either state or federal providing that you can express real damages. You can file a case not seeking damages where the remedy is to remove the offensive/damaging statement.
Filing a state claim is relatively affordable to defend as compared to a federal claim. A federal claim can easily cost $10,000 to defend even if they are able to defend the claim. Federal claims often survive for about 2 years before going to trial. While this seems to the defendant to be an advantage, it is not. The entity filing the claim has very little cost outlay while the defender has monthly costs that really add up. In the end, valid case or not, it is far better to settle a federal claim than fight one.
Your claim must be valid, provable, provide a no-cost option, have real damages, or have a no cost remedy. In additionally, no other action can be taken that would damage the case.
A kind letter goes a long way. You can ask to have the offensive post removed providing that you can make a valid case, can motivate the entity to act in your favor, and potentially remedy any previous harm, loss, or damages (assuming that they were not happy with some kind of service or product). However, do not at any point admit to fault on your behalf in case you do need to take a legal action. Be sweet. You motivation si to solve a roblem with a win win for everybody. You will want to contact every contact name and address available. This would be found on the website, using whois data, and possibly potentially contacting the web host who will often/sometimes act as a middle man if contact information is scant.