I received a question recently about excluding certain items from the search results and thought it was worth a post. There are two main options that I thought of when asked.

Firstly, editing the XSL on the Core Results web part that controls the format of the returned results. This works fine for actually hiding the items, for example, you can wrap the results display part of the XML in an xsl:if to exclude the current item:

<xsl:if test="contenttype='Contract'">

(assuming you have contenttype included in the Selected Columns XML within the Core Results web part)

The reason why this is not such a great idea is that the web part that displays the results statistics will still include the items in the count and seeing as you cannot access the hidden control that binds the web parts together and can't change the output of the Statistics web part you cannot change that.

The best way is to limit the results based on Scope. Either by creating a separate Scope that can be used when searching or by modifying the default Scope used, ie. All Sites.

First you need to make sure that the Managed Property you want to limit the Scope with is allowed to be used in scopes. To do this go to Central Admin, Shared Services Admin for the SSP in question, Search Settings, Meta-data Property Mappings. Then edit the Managed Property and check the 'Allow this property to be used in scopes' check box:

Next, you need to add a new rule to the Scope to Exclude the items based on the value for the Managed Property. Go back to Search Settings, Click on View Scopes and then edit the scope you want to change. Click Add new Rule and create a Property Query rule that is set the to Exclude items where the property you modified above is equal to the value you want to exclude. For example to exclude Folders from search results:

(note that if you're interested in excluding folders, the above works in SharePoint, but you'll need to do something more for file shares - see Scott Jamisons's post)

Then make sure that you run the Update Scopes option so that the change to Scope takes effect.

I'm not 100% sure, but I think that if you have not had a Scope based on this Managed Property (or it might be if you enable the Managed Property for use in Scopes) you may need to do a Full Crawl to have this all work properly.

Also, if you check the 'Show Scope Picker' option in the Advanced Search Box web part, and you have created a new scope for the Exclusion to apply to, the user can select that Scope when searching. 

Be aware when doing all this that the will be a price to pay somewhere if you have a lot of Scopes defined in this way (even if it's only in confusing the user!), and that if you need really custom restrictions, perhaps you are better off with a custom search solution tailored for the way that the users need to search!