Jeff Volpe has a signal attenuator that some people have claimed works well for them. I have no experience with it, but it's the only one I know of on the market.
http://jvde.us/xtb/XTB-ANR_description.htmIf it's just a low signal strength, you could add a higher performance coupler/repeater/signal booster. I tried the passive one from X10 and was underwhelmed. Others recommended Jeff's XTB-IIR, and after installing it I'd recommend it to others.
http://jvde.us//xtb/XTB-IIR.pdfHe also has some high performance filters (have no personal experience with these, but his other tools are great so these should be nice, too):
http://jvde.us//xtb/XTB-F10_description.htmIf you can figure out the source of the noise/signal sucking, you can add a filter on that device. If it's a plug-in type, you can use pretty much any filter you can find. I'm preferential to the SmartHome FilterLinc (Ebay has some good deals periodically), but there are some other units that would work fine. If the device is wired in (furnace, water heater, oven, etc), then you would need to use a wired type filter. That can be more challenging since those filters only go up to 20A and many of those devices are much higher. If you are familiar with electronics, you can probably solve the problem by putting the filter on the logic board rather than the whole unit, but that's not something I would do as a first step.