Sounds like you've your content framed within their site.

<frameset cols="25%,75%">
<frame src="MYheader.htm">
<frame src="http://www.yourdomain.com">
</frameset>

Easiest way off the top of my head to prevent this is to use javascripts and target tag.

You can use javascript to reload your domain to the top over the parent if loaded within a frame and then add target="parent" to reduce the impact on those that don't have javascript available.

This should be sufficient to get rid of any spoofers.

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@seopo | Estate Agent Website Design | Property Social Network