Does a creature with reach have to block a creature with flying if a ground creature is also attacking?

Blocking is voluntary. CR 509.1 states that the defending player chooses which, if any, of their untapped creatures will block — there is no rule that compels a creature to block anything. A creature with reach (CR 702.17) simply gains the ability to block creatures with flying; it does not gain an obligation to do so.

Reach (CR 702.17a) reads: 'A creature with reach can block creatures with flying.' This expands the list of creatures it is eligible to block compared to a normal ground creature, but eligibility is not the same as requirement. The defending player retains full discretion over whether any given creature blocks, and which attacker it blocks.

So if two creatures are attacking — one with flying and one without — a creature with reach can block either one, both (if it has some multi-blocking ability or there are multiple blockers), or neither. None of those choices is forced by the rules.

Concrete example: Your opponent attacks with a 2/2 Grizzly Bears (ground) and a 2/2 Wind Drake (flying). You control a 3/3 Giant Spider (reach). You may block the Wind Drake, block the Grizzly Bears, or not block at all. The presence of the flying attacker does not obligate your Giant Spider to block it.

The only exceptions would be unusual card effects that specifically force blocks (e.g., 'All creatures able to block CARDNAME do so,' CR 509.1b), which override normal choice — but reach itself is not such an effect.