Safe to kill SPID as long as its a SELECT
Ultimately it will depend on whether your website is coded to handle this happening. I have seen blocking occur when a select statement is issued within the context of a transaction which then locks a considerable amount of rows until the transaction is complete. If your website is in the middle of placing an order or billing somebody's credit card and you kill it bad things could happen if the website is not coded to handle it properly. To be sure consult the developers of the website - they would be your best resource in understanding how it will cope in situations like this.