Bugman Pest Control refusing to honour guarantee

Bugman treated our house for termites and had to return three times for additional treatments. There is still a problem with termites and they say they will come out at R1000 per visit. This is despite providing us with a 5 year guarantee on the treated area. My understanding is that when you buy a company you undertake to honour all contracts and prior commitments of the company Trevor, the current MD, was very adamant that he is not going to honour the guarantee given by the previous owner as according to him it should not have been given owing to the degree of infestation. We were very clear about the extent of the problem and emphasized it repeatedly to Ben the previous owner.