First, read the group's guidelines. There are some that allow links as long as they do not look sales-y.
If you put captions, make them simple yet enticing. Still not sales-y. They'd only consider spam if you "over-post," like when you place 10 posts per hour.
Your posts should also sound human, not as if a bot is doing it.
If they cannot accept you anymore, go find another group instead.