I was told in Ontario anyway, that no matter what signs are posted or what contract is signed, you can not sign away your rights. However, it does make it more difficult for the suitor to say they didn't know the risks.
I live in ontario. In 99% of the barns i've been to, they require you to sign a risk waiver basically saying you understand the risks involved and you are to participate at your own risk, before riding a horse. 5% of those barns require you to sign the risk waiver before stepping into the barn.
The way the waivers are worded is that once you sign that sheet of paper you agree that any injury is your fault and the barn owners/managers/horse owners cannot be held liable in any way shape or form.
^^ Yup, lots and lots of waivers are signed like that for many different events, but in the end, the person that signs can not legally sign away their rights. It does help to limit the damage payable though. That's also why service providers have to have a million different qualifications for everything now -- otherwise they can't get insurance.
You have the right to attempt a dangerous sport even with the understanding that you could be injured, because it is dangerous. Then you have the right to sue someone because you were injured while performing a dangerous sport, that you knew was dangerous?