Joe's answer is very good, but I would like to add the following, which should help explain the reason for such clauses.
In most cases these clauses are added to job ads to try and help candidates self-select out of a position.
Most employers get inundated with tens or hundreds of resumes per job offering. By HR filtering out those with less than X years commercial experience in Y technology, they are hoping to cut down on the number of resumes they have to filter through to find the three or four stand-outs that they wish to interview.
A clause like this acts as short-hand to say "don't apply if you don't have any experience".
Generally speaking, there are two types of people who look at resumes:
1) HR drones who know nothing about the actual job requirements; and
2) Managers (particularly at smaller companies) who need someone to lighten their burden or expand their team.
In the case of type 1, you are unlikely to be able to get past a filter like this if you don't have the prerequisite experience.
However, in the case of type 2, you may be able to get past this filter if you can provide good justification as to why you want the job and why you'll be a good fit. Many times demonstrating an understanding of the business you are applying to is the best thing you can do to move to the interview stage, even if you don't exactly meet the experience laundry list. The cover letter is the best place for this explanation, but you can still pad your resume if you have done something to make you stand out from the crowd (like made open source contributions).
In any case, always assume that the person reading this is a type 2 person, just in case they are. If your resume stands out you still have a very good chance of getting a job even if you haven't completed the laundry list.