Adverb

The adverb 'completely' should be changed to the adjectve 'complete'. Adverbs modify verbs, adjectives, and other adverbs. 'Rubbish' is a noun and should be modified by an adjective.

I completely agree with Lewis on this point, and have happily up-voted him.  But in fact with certain kinds of negative nouns, like rubbish and nonsense, at least in casual spoken British/Canadian English, it is becoming more and more common to hear adverbs modifying them.  Like Lewis, I pretty much come down on the prescriptivist side (complete rubbish only!), but...might this be one of those grammar points that is tottering on the precipice of obscurity, like who/whom?  I think it is not as far along as who/whom, and so I would definitely tend to insist on the adjective + noun pattern.  But... 

I've been like a dog with a bone on this question, and can now add to what Lewis and I have already said. In fact, in informal British English, "rubbish" IS used as an adjective. "She's rubbish at cards," for example. This lengthy exchange put me onto the adjectival usage: http://english.stackexchange.com/questions/39736/use-of-completely-rubbish, and I checked the Oxford online dictionary to confirm it.

Thus, in informal British English "completely rubbish" would be grammatical.