Not possible to "block" lactic acid production entirely.
While cycling at any intensity your body is producing (carbohydrate breakdown, aka glycolysis) and removing lactic acid from your bloodstream. At increasing intensities, the lactate is being produced faster than your removal mechanisms. No supplement is going to alleviate this biological process.
Are you aware that "lactic acid" is not what actually slows you down after you've been going hard for a sufficient time?
Lactate, in and of itself, is actually used by muscles as a source of fuel and is merely glucose that's been broken down.
Still, there's nothing in a bottle/pill that's going to "reduce" the biological process of lactate formation other than a reduction in exercise intensity. You can do the work necessary that will allow your body's muscles to better utilize the lactate and buffer the resultant hydrogen and phosphate ion production from glycolysis, but there are no shortcuts out there...that simple.
Not buying the theory behind the above...ymmv
Carnosine is already found in high concentrations in skeletal muscle such that additional supplementation to raise the levels even further is like putting on a belt when you're already wearing suspenders...
There is also Sodium bicarbonate which is an acid buffer. I have used it Couldn't swallow enough for It to have a positive effect and in that quantity it will make you **** yourself as or becomes very fizzy and almost carbonated drink like
I used the Twinlabs acid buffer back in the 90's, Phosfuel I think it was called but Sodium bicarb was the active ingredient. It worked for me. Their was a downside and that comes along with intake of two much sodium in any form, Yellowcat mentioned it... diarrhea. I literally had to run over to the side of the road and drop my drawers one morning before a race in Prospect Park so I wouldn't **** myself. There was no question that I was able to run the small hill in Prospect at least a gear or two bigger with the same perceived effort. Placebo, maybe. The verdict on acid buffers is still not entirely in but Eastern block coaches were giving this stuff to their athletes back in the 60's. I'm racing again and used a product for a couple weeks last year called Extreme Endurance, also supposedly a lactate buffer. I didn't have nearly the effect I remember with the Twinlabs product. Don't really matter though - I put in more (and often harder) miles these days and am a much stronger rider. Plus my races are longer these days and shitting mid-race is not an option. /img/vbsmilies/smilies/biggrin.gif