How do you free a control valve handle not touched for 30 years

You didn't tells the model but most of the handles pull off after removing the small cap on the handle and then the screw to the stem. Being that old, and probably a bit corroded, you will likely need some WD 40 or similar to loosen the handle from the stem. You can get a handle puller. If the old handle is really stuck, you may need to carefully cut the old one off, not damaging the stem,and get a replacement handle

Usually, you can remove it with little effort. Remove the screw and wiggle the handle out. Tap handle with screwdriver handle - up/down & left/right side - while pulling on the handle and slowly wiggling it out. Eventually, it will go. If not, break the handle, do the repair, and install brand new handle. Symmons handles are about $10.00 in Home Depot

Milo