Is it true that a calcium supplement reduces the absorption of some vitamins and minerals (like in a multivitamin for example)? if so which ones?

Possibly: Yes it is true that calcium can affect absorption of some vitamins but also calcium often isn't that easily absorbed either. Remember, a lot depends on whether you are taking supplements on an empty stomach or not. Actually, it is better to take vitamins after a decent meal as the proteins and fats in food will aid in absorption of supplements and vitamins.