There is a rather extreme measure, which should work: run a virtual machine on your side. That machine will power some minimal Linux system with X11. You do the
ssh from that machine. That way, if the worst happens, the evil people on the remote machine may compromise your virtual machine, but just the virtual machine.
(Beware of "guest additions" from the VM: this is the kind of extension included in the guest system so that things like copy-paste work between host and guest; such a system may help a compromised guest X11 obtaining information from the host X11. Don't install such extensions. For extra safety, deactivate support for guest additions in the VM engine.)
A lighter version involves Xnest: it is a X11 server that uses for its display a window within an outer X11 server. This will be enough to protect your other applications from X11-based attacks (e.g. sending synthetic events), but not if the attacker succeeds at compromising the connected X11 server through some buffer overflow or a similar hole.