Can you legally block access to another's property if your property is the only way they can get to theirs?
Depends....not enough facts here...:
If their property is "landlocked", meaning it has no access from
a public right of way like a road, there are a number of
factors:
1. Was your land and the neighbor's land originally one tract?
If so when it was divided into two ownership parcels an easement
may have been recorded. Even if one was not recorded if access had
been established previously and the land was split, an easement may
be implied.
2. If the use has been there long enough, the neighbor may have
rights of adverse possession in your land and if you don't
negotiate something that is mutually beneficial you could lose
permanently some rights in your land.
3. Some states have laws that REQUIRE the owner of land that is
"blocking" another parcel from a public right of way to allow an
access easement, and the landlocked land owner can go to court to
enforce it (Wisconsin as an example has such a law).
4. Why do you want to block access? Are you looking for a
lawsuit? Or would access somehow materially interfere with use of
your land? It may be better to negotiate an easement that could
require the neighbor to pay you or to improve the access route with
a paved road that you might be able to use too.