Assuming your continuous integration server is using MSBuild to build the Office 2007 Add-in's a quick workaround will be to execute a Build target followed by a VSTOClean target.
You can achieve this by creating a MSBuild project file (master.proj) that controls the build process as illustrated in the following example:
<Project
DefaultTargets="Build"
xmlns="http://schemas.microsoft.com/developer/msbuild/2003" >
<PropertyGroup>
<WorkingDirectory>C:\BASE_DIR\</WorkingDirectory>
</PropertyGroup>
<ItemGroup>
<VstoProject Include = "$(WorkingDirectory)OfficeAddInProject1.csproj"/>
<VstoProject Include = "$(WorkingDirectory)OfficeAddInProject1.csproj"/>
</ItemGroup>
<Target Name="Build">
<MSBuild Projects="@(VstoProject)" Targets="Build;VSTOClean" />
</Target>
</Project>
Update:
If cleaning after is not enough for you you can stop the registration process by overriding the property (VSTO_ProjectType). For a office add-in this property is set to Application which forces the registration process to take place. By setting it to a custom value you disable the registration. These examples explicitly list which projects to build but you can test them also by specifying a solution file. However the CI server must use MSBuild directly and not VS to perform the builds.
<Project
DefaultTargets="Build"
xmlns="http://schemas.microsoft.com/developer/msbuild/2003" >
<PropertyGroup>
<WorkingDirectory>C:\BASE_DIR\</WorkingDirectory>
</PropertyGroup>
<ItemGroup>
<VstoProject Include = "$(WorkingDirectory)OfficeAddInProject1.csproj"/>
<VstoProject Include = "$(WorkingDirectory)OfficeAddInProject1.csproj"/>
</ItemGroup>
<Target Name="Build">
<MSBuild
Projects="@(VstoProject)"
Targets="Build"
Properties="VSTO_ProjectType=Custom" />
</Target>
</Project>