The headline on that page is "Ethics Group Asks Pelosi to Seek Rep. William Jefferson's Resignation From Congress." Seek is not the same thing as make.
Pelosi has no power to force Jefferson to resign, though she can strip him of his seniority and all his committee assignments and basically make him feel about as welcome as a pork rind at a bar mitzvah.
The House can, by a simple majority vote, expel any member, but that traditionally isn't done without a conviction. Most convicted members of Congress resign immediately after being convicted, if not before; the only recent exception I can remember is Jim Traficant, who was expelled and is currently in federal prison.