[Scenario]

You need to name the timer when you first create it. Here's an example:

You'll need to of course add a new timer variable at the top of your script to make it global. So... new Timer:OneSecTimer[MAX_PLAYERS];

This is going to have to be under the first command: OneSecTimer[playerid] = repeat PlayerData(playerid);

Then in the killtimer command, you'll just need to have: stop OneSecTimer[playerid];

See how that works for you. If you have any questions, please ask!