There is possibly an easier way to block spells, but we don't see what it is.
Lockhart was going to teach the second years in Dueling Club how to block spells, and Professor Snape didn't tell him that doing this would be impossible, so it's likely that there's a relatively easy way to block spells.
“I think I’d better teach you how to block unfriendly spells,’ said Lockhart, standing flustered in the midst of the hall. He glanced at Snape, whose black eyes glinted, and looked quickly away. ‘Let’s have a volunteer pair – Longbottom and Finch-Fletchley, how about you?” - Harry Potter and the Chamber of Secrets, Chapter 11 (The Duelling Club)
There's also something called hex-deflection, which Hogwarts teaches to fourth-years.
“Hermione hung back in their next Defence Against the Dark Arts lesson to ask Professor Moody something. The rest of the class were very eager to leave; Moody had given them such a rigorous test of hex-deflection that many of them were nursing small injuries.” - Harry Potter and the Goblet of Fire, Chapter 28 (The Madness of Mr. Crouch)
If they can't cast Protego, they can also dodge or block it with a strong enough object.
Another option for defense is simply avoiding or blocking the spells with a physical object rather than a conjured shield. This would work for wizards incapable of casting a Shield Charm, and even skilled wizards like Dumbledore use this method of defense sometimes.
Sirius dodges a a spell, although in his case it doesn't help him for long:
“Only one pair was still battling, apparently unaware of the new arrival. Harry saw Sirius duck Bellatrix’s jet of red light: he was laughing at her.
‘Come on, you can do better than that!’ he yelled, his voice echoing around the cavernous room.
The second jet of light hit him squarely on the chest.
The laughter had not quite died from his face, but his eyes widened in shock.” - Harry Potter and the Order of the Phoenix, Chapter 35 (Beyond the Veil)
Even Dumbledore sometimes just avoids the spell instead of conjuring a shield, and he's certainly capable of creating shields:
“Voldemort raised his wand and another jet of green light streaked at Dumbledore, who turned and was gone in a whirling of his cloak. Next second, he had reappeared behind Voldemort and waved his wand towards the remnants of the fountain.” - Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)
He's also able to block the Killing Curse from hitting him with a statue:
“Another jet of green light flew from behind the silver shield. This time it was the one-armed centaur, galloping in front of Dumbledore, that took the blast and shattered into a hundred pieces, but before the fragments had even hit the floor, Dumbledore had drawn back his wand and waved it as though brandishing a whip.” - Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)
Expelliarmus is also a common and simple defensive spell, but it doesn't block spells.
“Right,’ said Harry, when she had sat down again, ‘shall we get practising then? I was thinking, the first thing we should do is Expelliarmus, you know, the Disarming Charm. I know it’s pretty basic but I’ve found it really useful –’
‘Oh, please,’ said Zacharias Smith, rolling his eyes and folding his arms. ‘I don’t think Expelliarmus is exactly going to help us against You-Know-Who, do you?’
‘I’ve used it against him,’ said Harry quietly. ‘It saved my life in June.” - Harry Potter and the Order of the Phoenix, Chapter 18 (Dumbledore's Army)