so long as you leave the cassette on.

geared bikes have dished wheels with hubs that have threads to spin on a cassette, so you'd need a new hub, and a re-dished wheel in order to truly convert to a fixie or a single speed. And for an old wheel, it's not worth it, probably better to buy a new wheel set.

if you leave the cassette on (the gears basically) then you don't need a new cassette, new hub, or redished wheel, but you're at the mercy of the chain line.

Some cassettes, usually newer ones, will let you take off the other cogs and put spacers in, so you can move the cog around using spacers to fill in the gaps, but thats only if your cassette is built like that. Mine wasn't.