Tony Verrier and Shaun Lynn, senior executives at BCG Partners, were found guilty of conspiring against the interdealer broker by a judge at the Royal Courts of Justice, London.
The former is thought to have led a conspiracy to steal staff from Tullet Prebon after he joined the company in January 2009.
BCG sought to acquire as many as 90 staff members by using false constructive dismissal claims to remove employees from contracts quicker than they should have been entitled.
Evidence which could have incriminated the company was deleted from IT systems while BlackBerryâs containing data were deliberately lost, the judge found.
Mr Justice Jack said: âI am satisfied that it was Mr Verrier's gambit to 'lose' BlackBerrys whenever he thought they might contain inconvenient material, and that his instructions were the cause of at least some of the mobiles being lost.
âI am satisfied that the inaccessibility of the contents of his last BlackBerry due to a missing password was a deliberate ploy."
Tullett Prebon has been granted the right to claim damages from BGC and is expected to seek $500 million from the firm.
By Jim Ottewill