What does it say about parking spaces in the documentation relating to the purchase of your house? Is it allocated or unallocated parking spaces?
Unless it specifically states that parking is allocated (whereby you should have been told which particular space is yours), then these people have no right to put up this reserved sign.
Given past experiences where this was an issue (and it can be an extremely touchy issue for some people, and especially I'm guessing people who feel the need to put up "reserved" signs), I'd be very wary about approaching this situation with them at all.
Personally speaking, I would remove the signs in the middle of the night. There's no reason (I'm presuming from above) that they should be there, so there can be no complaints if they're taken down.
Whether or not you park there is one thing, but it does leave it open for other people to park there. You can always give the sign back to them people concerned at a later date so you're not accused of theft. Even post it back to them so they won't know it's you.
(crossed with MF1 above).