You mean apart from enclosing them in an airtight acrylic shield and posting a guard dog around them day and night? It is very difficult to keep squirrels away from plants, because they are extremely agile climbers and are very good at working their way around defensive devices. They are ubiquitous in suburbia. When squirrels are focused on food, they can be very persistent. You need to eliminate any pathways they can take to the plant, because once the squirrels get to the plant, they can climb it quite easily and cut the seed head off with its teeth. Placing a shield around the stalk might discourage some (e.g., large flat disk or inverted can around the stalk, supported by a stake to the ground), but may be ineffective, because the squirrels may just cut off the stalk closer to the ground. These are used to keep squirrels out of bird feeders, with mixed success, but something similar might be worth a try. You may get some relief by building a frame around the plant(s) and attaching bird netting to it. The netting should be far enough away to avoid touching the plant. This is designed to keep birds away from fruiting plants. The netting is made from plastic and has a fairly small mesh - too small for a squirrel to squeeze through. This may only slow down the squirrels, because they can get through plastic mesh with their teeth. If you can find a similar metal mesh, you might have more success. Another possibility is to distract the squirrels by providing another source of seed. The squirrels may prefer to go after a dish of seed, rather than the plant itself. The seed supply would need to be close enough to the plant to provide an alternative, but far enough away to keep their tiny little minds focused on the bowl and not the plant. You might also want to plant one or two extra plants to sacrifice to the squirrels. You can cut off their seed heads to feed the squirrels in place of a dish of seed. Killing the squirrels or trapping and relocating them are ineffective means of control, because others will simply move into the vacated territory.