htaccess mod_rewrite for specific starting substrings only

OK, when reading this question please ignore the fact that the developers should be fixing their out of date plugins... this is another battle being fought elsewhere.

We are trying to block querystrings in the format of mydomainname.com/?q=anytexthere

I have successfully used this code in the htaccess file:

RewriteCond %{QUERY_STRING} \bq=\b [NC]
RewriteRule ^ - [F]

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But this unfortunately stops timthumb.php requests from working, I assume because the timthumb string includes ;q=21 (for example).

So, given that we only want to block requests that start with /?q=anytexthere how would we modify the RewriteCond ?

Help appreciated.