Sorry but it cannot under law be considered as 'theft', for a situation to be considered as theft the person must have taken something from someone with the purpose of 'permanantly' depriving the owner of it. A clamper does not intend to 'permanantly' deprive the owner of his/her property..You might check that the clamper is a registered person and has the badge to prove it, also they must display the badge on their person, and show the badge upon request, otherwise they cannot conduct their business