Can too much calcium during pregnacy cause hard-headed babies?

No. Your calcium intake is not likely to affect the pregnancy regardless of whether it is too high or too low. The placenta does a very good job of providing what the baby needs and protecting it from what it doesn't need.
Unlikely to be a pbm. The maternal placental unit and it's complex relationship draws on mothers nutrition and body to make sure the fetus gets what it needs to grow & develop. The mothers GI tract will only absorb so much calcium, before it begins to cast it off into urine or never absorb it from the gut.There are complex systems that keep this in check. Baby will do fine.