Remove old non-indexed levers for gear changing to make "fixed" speed bike?
If I understand correctly you have slipping shifting levers that let the derailleurs move without your intervening. A non-indexed shifter slips because the friction setting is too low.

Friction of the lever is easily adjusted by a screw at the side of the lever. If it still slips after tightening you'll unscrew the thing taking care to remember the correct order of items when is comes apart. (Take a photo maybe to assist your memory.) Then clean the bits to remove any grease and reassemble. It usually helps. At least it did in the old days before indexed gears.