how to clear only the query plans from the current session?
Marcello, it seems you have the answer in your question, albeit with one minor modification.

The query:

SELECT cp.plan_handle, st.[text]
FROM sys.dm_exec_cached_plans AS cp 
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st
WHERE [text] LIKE N'%/* GetOnlineSearchResultsMonday %';


contains the term '%/* GetOnlineSearchResultsMondy %', which is a wild-card search used to locate the specific plan to remove from the cache.  If you ran it with a search term contained within the query you are attempting to tune, it should return a plan handle that can then be used with DBCC FREEPROCCACHE:

-- Remove the specific plan from the cache using the plan handle
DBCC FREEPROCCACHE (0x05000800F7BA926C40C15055070000000000000000000000);


I would suggest perhaps using the following to identify the query:

SELECT cp.plan_handle, st.[text]
FROM sys.dm_exec_cached_plans AS cp 
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st
WHERE [text] LIKE N'%CASE WHEN opgv.lngpaymentID IS NULL THEN NULL%';


Assuming that string is not used in other queries, the results should only return a few rows.  You would then look at the st.[text] column to ensure you have the correct plan_handle, then use that plan_handle with DBCC FREEPROCCACHE to remove it from the plan cache.

As a side-note, I would be very careful running an SSMS session against a live production system - it can be a recipe for disaster, since it is very easy to forget your are working on the production server.