Oops sorry guys I was afk and missed all this. Apologies in advance if my tangential knowledge is of little use.
The problem isn't killing the microbes, it is getting rid of the existing biofilms they form on surfaces. For this, you can try two approaches: Try to get the pH of the solution to a point that the microbes don't adhere well any more. Low pH is better for this (dilute nitric acid at ~ 0.01 or 0.001 M would be what I'd try first). The other option is to add surfactants to solubilize the microbial gunk. This is more or less what airspirit did with success.
If I were dealing with this in the lab, I'd just use a sonifier + solvents to clean the components. Wouldn't be worth the effort to clean tubing this way though (sonifiers usually aren't so large).
As for an effective long term biocide, look at sodium azide. It is not nice stuff to work with (wear gloves!) but it is packed along with dialysis tubing so that it can be stored moist (it cracks when it dries out) over many months. I cannot at this moment comment on its chemical activity but take a look at it.
H2O2 and bleach are both pretty potent oxidizers. You'll end up with a Cu(OH)2 layer on your block with these pretty quickly.
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