How to clear ad hoc queries from plan cache?
So you just want to clear the Ad-hoc query plans and still dont want to clear the whole procedure cache. What you are asking is there in Clearing Your Ad-hoc SQL Plans While Keeping Your SP Plans Intact
The blog asks you to run
DBCC FREESYSTEMCACHE('SQL Plans')
As per the blog

The procedure cache actually consists of 4 distinct cache stores that hold different types of plans. Those cache stores are:

CACHESTORE_OBJCP - these are &quot;Object Plans&quot; - stored procedures, functions and triggers. Generally, the good stuff.
CACHESTORE_SQLCP - these are &quot;SQL Plans&quot; - ad-hoc SQL statements (including parameterized ones) and prepared statements. This is the
stuff that we are after.
CACHESTORE_PHDR - so-called &quot;Bound Trees&quot; for views, constraints and defaults. Irrelevant for the problem discussed.
CACHESTORE_XPROC - not really execution plans but rather pointers to the entry points of your extended SPs.


So you can see selectively clearing the SQLCP would remove prepared and ad-hoc plans. I tested this on my system.
Ran query
select objtype, 
count(*) as number_of_plans, 
sum(cast(size_in_bytes as bigint))/1024/1024 as size_in_MBs,
avg(usecounts) as avg_use_count
from sys.dm_exec_cached_plans
--where objtype='adhoc'
group by objtype

And the output was

You can see the the picture has 1264 ad-hoc plans and 69 prepared statements.
Now I selectively clear the SQLCP using DBCC FREESYSTEMCACHE('SQL Plans') and rerun the query again which gave me below output

Now you can see the ad-hoc and prepared plans are 2 and 6 respectively. While other are NOT affected.