have you signed a contract saying they can use it in any publication?
If not and you own full copyright, threaten them with legal action unless they pay you.
(
HappyToast Groat froth,
Mon 17 Sep 2007, 11:29,
archived)
Of course, you'll lose a client if you threaten to sue them
so maybe you should use some tact rather than threatening legal action straight off the bat.
Perhaps "I'd rather you didn't put these in competitor's magazines" will work.
(
gronkpan @vomitinglarry.bsky.social,
Mon 17 Sep 2007, 11:31,
archived)