How can I stop a saddle from creaking in a spinning bike?
Possibility 1: Take the clamping system apart and clean the parts. Then reassemble with the metal to metal contact surfaces properly greased.
Possibility 2: Replace the saddle with one of better quality as the most probable cause for creaking could be the saddle itself, especially where the rails are lodged in the shell. If you want to go a step further and if the bike allows for this, replace the seatpost as well with one of better quality to get rid of the ancient one bolt clamping device.
The kind of clamping device that holds your saddle has come out of use and has been replaced by one-piece posts for mainly two reasons: weight, though this is not a problem on a spinning bike but also because the serrated surfaces work against each other through the dynamics of the moving cyclist, thus bringing in play into the system and requiring constant tightening, hence the squeak. They also permit incremental tilt adjustment only which is less precise than with two bolt seat-post heads.