Nimble Group Harassment without a case

How can one be protected against harassment from a debt collecting company without a liquidated claim or for that matter an allegation of debt which has not been proven nor established by fact? I understand that the call operator is simply phoning a list of numbers but despite numerous emails to this company the merits of the allegation has not been investigated or considered. The Nimblegroup have simply assumed that all claims bought over from Autopage are meritorious. Surely there is an obligation however on these companies to first review the facts of each case they take on to their books? In the mean time i am pestered at work on my work and mobile phone having to explain to a new call operator almost on a daily basis how the claim initiated by "predecessor" of this claim, that does not exist, is not valid. Can I take legal action against the call centre company?