Are lobsters cannibals?

Yes, some species of lobster can be cannibals, though this would never be the most important mode of feeding.

European and American clawed lobsters (Homarus gammarus and Homarus americanus), which are probably what most people will think of as 'the lobster', will prey upon smaller individuals of their own species if they are brought into close proximity. This can happen in lobster traps, for example, but in most cases the smaller individuals take care to avoid larger lobsters. Norway lobsters (Nephrops norvegicus), also known as scampi, langoustine, Dublin Bay prawn, ..., will certainly prey upon juveniles of their own species, and this is one factor thought to constrain the numbers of young added to the population when adults are dense on the ground.

Cannibalism may be less prevalent in rock lobsters (perhaps someone else can confirm this)? Some species will actually aggregate rather than avoid one another.