Both the influencer and the advertiser are independently responsible for the advertisement itself. Therefore, both can be held accountable, including by the Advertising Code Committee. In practice, however, the advertiser is always held accountable. This is also because it's often a competitor who finds an advertisement objectionable and files a complaint about it, or has a consumer file one.
As a result, all rulings and recommendations are published. Only if a party overseas data fails to comply will they be blacklisted. Legal action is also possible, and the Netherlands Authority for Consumers and Markets (ACM) may also take enforcement action, but this is rare.
Post-payment discount
A vlogger once showed how she went into a bed specialty store to pick out a bed. At that point, there was no contact or agreement with the store. Later, after the vlog had been published, the store offered a discount on a bed. The store liked the vlog and posted it on its website.
The Advertising Code Committee subsequently ruled that the advertising in question should have been mentioned in the vlog or in its YouTube description. They reached this conclusion, in part, because the video had been embedded by the bedding retailer.
Image from YouTube, where as an influencer you have to mention this in the description of your video if it is an advertisement.
It's important that clear agreements are made between advertisers and influencers. After all, it's essentially a contract of assignment between the client (advertiser) and the contractor (influencer), with or without an agency coordinating the process.
If an agency is involved, the advertiser will also need to make proper arrangements with them. For example, to shift some responsibilities, including liability for communication about advertising, to the agency.