OP's asking how to go about the illegal activity of price fixing?!
Price fixing is rife in our industry, aquatics, as the industry is very antiquated. Shop owners are used to VERY high margins compared to most other sectors. The advent of online shopping suddenly made it possible for online sellers to slash the margins and rocket their business skywards with ease. Thats basically how I got into it - It's hard to be sure, but I reckon we're number 3 online seller in the UK now. Took 3 years.
I understand the OP's issue, it's the age old (well approx 6 years old since internet shopping became big in my case) of a minority of retailer upsetting the majority that the OP relies upon.
It must be tempting to believe that you can afford to lose the minority of 'price slashers' if it secures the business of the majority of retailers that follow rrp. That's very short term thinking though. the OP must ask themselves what would happen if they shed the price slashers, and other suppliers find a diplomatic way to keep both parties happy(ish). The reality is, that the way retail works is changing, and effiecient, often distance selling is only going to get bigger. By fighting the trend you risk kicking yourself out of the market as a supplier.
within 5 years, especially given the economic situation, it's entirely possible many industries will start to see discount/online/distance sellers become the majority.
To be clear, I am not suggesting their is nothing the OP can do. Whilst it is true that it's illegal to close an account 'because retailers won't allow you to control their prices' it does happen everyday. The vast majority of our suppliers are actively price fixing, we all know it. No matter how it's dressed up. There is an amazing short-coming in the rule book however. Whilst you cannot close an account due to reselling price, you can in fact close an account for no given reason whatsoever. And no suitable level of policing exists to investigate your reasoning anyway. Not unless you're selling oil/gas on a multinational scale.
But like I say, it's easy to fix the short term problem the OP has by closing accounts, but the clever money is on coping with the transition in order to still be a relevant and competetive supplier in the long term. You can't be that supplier if your own competition take on the bulk of the new market share.
Also remember, it's not the bargain basement sellers that decide the price, it's the end customer. If the end customer wants to buy at low price, and it's possible to make a business work whilst selling at that price, the it's the entire industries job to reform it's ideas and make it so.
Price fixing, tut tut!