If someone signs a waiver regarding equine activities on my property and they release me from liability, can they still sue?

Unfortunately, there is no way to fully insulate yourself. A waiver helps, especially vis-a-vis injuries where you (or your family or staff) did not act recklessly or intentionally; and as a general rule, whenever anyone engages in an inherently risky endeavor--like riding; or skiing, parachuting, rock climbing, etc.--they accept certain of the ordinary risks of that activity and cannot successfully sue for them. For example, if somone's horse shies because it is stung by a bee and the rider falls, he or she could recover money from you, since that is an ordinary risk of riding. All that said, if you or your staff created dangerous conditions (e.g. did not maintain the fields properly or fill in known or obvious holes; or used horses that are known to be too wild to ride for all but the most expert riders), or act in some way recklessly, with gross negligence, or intentionally in some fashion that causes injury, you will still be liable for it. So waivers can reduce the risk of  liability, but cannot eliminate it. The best way to protect yourself would be to have adequate insurance for your activities or business.