Usually friction alone is enough to hold one end, since the thread is of a smaller diameter than the flange nut. Just make sure the thread is greased and the static shoulder dry. If yours is spinning first check that it's short enough and the thread is smooth all the way to the shoulder. Sometimes on old bolts the screw is a bit stretched, bent or has a damaged thread so it binds before threading all the way.

Also check that the ears and seat tube are OK and not pinching closed before the post is tight.

BTW, in addition to the tabbed bolts, some have built in serrations on the non-turning end, and there are also 2 key versions which take an allen key at each end. This is my preferred design because there's less chance of breakage if you hold the screw end and turn the nut. I have a few of these and can sell you one at a low cost if you know the length you need between the shoulders.