Comment by toast0

Not all TLD registries offer registry locks, but those run by Verisign (includes .com, .net), Affilias (lots of gtlds), and Neustar (.biz and .org), and according to a MarkMonitor slide deck, about 30 ccTLDs do.

You may need to switch to a registrar that will let you do it, but that's generally a matter of finding one and giving them enough money, which isn't really difficult. Look for a corporate/brand focused registrar and call their sales and see what the minimum spend is.

If the registry for your domain doesn't do any registry locks, then you would need to switch to a different TLD, which is difficult.