Is it possible to block a specific query, or return a static result?
As far as I can tell the answer to this question is no.  You can't block this SELECT at the database level, or rewrite it in versions before 5.7.6 (dev milestone 16 for the 5.7 release).

If the offending statement were an UPDATE, INSERT, or DELETE then you could use the tactics described in this StackOverflow answer to nullify the statement via a trigger.

As established in this answer (also on StackOverflow), you can't put a trigger on a SELECT statement.

Given the above is true, you will have to do one of three things:


Put a proxy like MySQL Proxy (that proxy was discovered by Dr. McKay) between the database and the application that will replace the query and return a cached/static result as suggested by jkavalik initially.
Wait for your support ticket to work it's way through WHMCS's bug system.
Figure out how to optimize this query until it isn't a problem anymore or option 1/2 are in place.


We can't help with option 1 or 2, but we can help you optimize your query.  Optimizing the query may bring the execution time down from tenfold, which will lessen your organization's pain, and make you look like a hero.  I vote you take this route while you are waiting for the bug ticket if you can't install the proxy or get it working.