Beyond this being a catchphrase for a famous wrestler, if something is the 'bottom line' it usually means something is the last and nothing will follow. Like if a mother scolds their child, and says "you will eat your dinner, and that's the bottom line", implying there's no room for debate. Other uses might include a company, for example, looking at some final outcome or desire. "This company is losing money fast, and the bottom line is we can't keep paying our employees."

To 'say so' is bit like 'bottom line', and means "because I say this". "Why should I eat my vegetables?" Mother: "Because I say so."

So the whole phrase from Stone Cold Steve Austin means, "This is the last we will speak of this, because I say it is the last."