'LOCK HIM UP, THROW AWAY THE KEY'... Says One of His Victims

One of R. Kelly's many victims thinks the feds' recommendation of 25 years in prison is too low, and instead, he should spend the rest of his days behind bars.

Lizzette Martinez  ... Kelly's more than earned a life sentence due to the fact he's hurt so many people over several decades.

She doesn't think his abuse will ever come to an end, because -- as federal prosecutors point out -- the guy's proven multiple times he's using his power to get what he wants, and he can't be given the chance to prey on women again.

Kitti Jones, another one of R. Kelly's victims, tells us she has no issue with the prosecutors' recommendation, however ... no amount of time will give her, and others, back their time or remove their trauma.

Both women were featured in the "Surviving R. Kelly" docuseries which sparked renewed prosecution, and ultimately, Kelly's downfall -- and they say they'll be present for his sentencing later this month.

As we reported, the U.S. Attorney's Office won a conviction against Kelly last September, and just filed its recommendation for his sentencing -- saying there's a laundry list of reasons to send him away for "in excess of 25 years."

Keep in mind, Kelly is still facing another federal case out of Illinois, plus a state trial in Minnesota.