Here's a shocker -- the most powerful talent agency on the planet refused to negotiate with a man who accused them of stealing his TV show idea and giving it to Shaquille O'Neal ... so now that dude has filed a lawsuit.

It's all over the show "Shaq Vs." A sports writer named Todd Gallagher claims the William Morris Endeavor Agency stole his idea about a show in which he competes against pro athletes in different sports -- and turned it into "Shaq Vs". ... a show featuring Shaq competing against pro athletes in different sports.

Todd had fired off a letter to the agents back in August in hopes of a settlement, but they refused ... so today he filed a $5,000,000 lawsuit in L.A. County Superior Court.

For the record -- Shaq has stated publicly that he came up with the idea all by himself.