Mister C,
If, within the GUI you can disable Attack Signatures from the policy itself, and this won't affect other policies that have the signature in its database. Open your policy and go to Application Security - Attack Signatures - Attack Signatures list. Search for the signature and click on it. Uncheck the Enabled box. Apply Policy.
On a seperate note, an Attack signature being triggered is different from other violations, say ASM cookie highjacking. Both violation types have their own Learn, alarm and blocking settings.
Hope this helps,
N