I have come across such restrictions in contracts in the past:
(1) As a consultant working on a client site, the contract between client and consulting company forbids the client from hiring consulting staff.
(2) When accepting redundancy packages from Employer A, unable to return to full time work with this Employer for at least 12 months
18 months seems a little long, but it does sound plausible on the basis that Company B paid Company A for your customer base and the skill sets, experience and industry knowledge of the TUPE staff. If Company A can just hire back those staff, Company B are losing that.