From the last publicised version of how Phorm/WebWise was to work, there are two cookies, one was an 'opt-out' cookie, so by deleting it you would be immediately opting yourself back into the targeted ads system. Another cookie was used to store your profiling info but if you delete that it will be replaced by the Phorm/WebWise system next pass through.
The problem that people should focus on is not the ads themselves, it's the 'man-in-the-middle'-like nature of what Phorm/WebWise does. It sits at the ISP, copying your page requests and responses, sifting them for keywords, which are then used to build a persistent profile.
If you choose not to view on-line ads, or be tracked, then you can take steps to block such actions by the likes of Google, etc, (via AdBlock, NoScript, etc) or use an alternative service (such as Scroogle so not even your IP is tracked), but you cannot avoid the snooping by Phorm/WebWise short of sending all of your traffic encrypted as everything goes through Phorm/WebWise kit at the ISP even if you opt-out.
No real detail has been given about how such data passing through their system is analysed, apart from assertions that they will not keep/use numbers over a certain length (that might be credit cards) and that they cannot view HTTPS traffic. They also promise not to keep data for opted-out customers, although initial reports said the data would still be analysed. There has also been inconsistent data given about how the system works, whether data is actually stored before processing, who will have access to the data at what stage, etc.
Phorm is the new name of a primarily Russian-based company formerly known as 121Media who previously produced software branded as spyware, and a rootkit, which they stopped distributing when the CDT in the US raised a formal complaint for deceptive behaviour.
Do you really want a company like that having access to your data, all of your browsing data, whether you opt-out or not?
Do you also want to use an ISP that has lied about using this system in trials, misled the public as to it's purpose, and now it seems operated without proper legal advice in the early stages?