*Sigh* CRB FUD
While I certainly don't disagree with many of the reasons for not giving a DNA sample in this case (mainly because as others have pointed out it would be a complete waste of time and money), the fact that it may show up on a CRB check is not one of them. There is so much rubbish spouted about CRB checks on these pages I feel I have to post to correct some of the myths. For the record I work within a youth service as a youth worker, I have been through the eCRB process and seen and consulted with other managers over other prospective youth workers and their eCRB checks.
First all, just because you have entries on your CRB check that is by no means a barrier to employement with children or vulnerable adults. I've known youth workers with CRB checks as long as your arm (one I've seen stretched over several pages). A CRB check does not say 'do not employ this person' (unlike the now cancelled vetting and barring scheme) it merely lists the stuff held on your file. It is then up to the judgement of the employer to employ you or not, based on the contents of the CRB check.
So for example, petty offences committed as a youth will be listed on the CRB check, but, as an example, if they were committed 10 years ago and the person has been well behaved since then they won't be seen as an issue. In fact, this in the youth service, this sort of life experience is very useful, especially if you may be working with young people who are currently getting into trouble with the law.
Driving offences will also show up, but as long as they are relatively minor they will be no barrier at all - although you may not be able to drive the organisations minibus, for example - again, that is down to the judgement of the employer.
If there's a record of you giving a DNA sample (I'm not even sure if this will appear on a CRB check) it might be querried ('Oh, it says you gave a DNA sample, what was that about', 'I volunteered when the police put a call out to help with their enquiries'), again, it won't be a barrier to any form of employment at all.
Anything related to violent or sexual crimes will almost certainly be a barrier to employment, and quite rightly.
The CRB system is not perfect - it's a pain in the arse having to be checked for every organisation you may work for and it can be expensive for the employers carrying out the checks, but it's by no means anything to be feared or paranoid about.