Evenin' all,
at the risk of putting the cat firmly back amongst the pigeons (or should I say Dragon amonst the Lakemen), I believe the bridge as shown in the drawing of Laketown (somewhere a few pages ago, you know, the first one) is easily 'cut-able'.
From the side view illustration the bridge seems impressively stoutly constructed. However, no top-view is shown. Therefore it is as likely as not that the 'road surface' along the bridge is made of big planks (I can see no other practical alternative). By all means good solid planks the size of railway sleepers indeed, but planks all the same.
Now IF these are laid across the supporting beams and not 'tied in' to the rest of the structure by being jointed or nailed (which is, I think, entirely plausible, for obvious defensive purposes), then you could quite easily get a gang of chaps, or indeed chapesses, to lever them up with crowbars and chuck them over the side of the bridge. One could even simply drag the planks back to the town-end if desired.
Although this would not stop individual people precariously making their way across the framework in single file (such as at Worcester in 1651), I think that it would make it very difficult for Smaug to get across. Therefore the bridge would be 'cut' (in the same way as one 'cuts' a road) and the bridge planks would be 'thrown down'. Et voila! From Smaug's point of view the town is now effectively an island.
PS. Is someone running this thread for a bet?