PowerShell is an option, and one I would go for if this will be a recurring need.
Here is an option using some T-SQL and the SSMS GUI scripting features that could get you there pretty fast:
1. Backup your database and restore it to a test instance.
2. Run this SQL on the restored database to get rid of objects you will not
DECLARE @sql NVARCHAR(MAX);
SET @sql = N'';
SELECT @sql = @sql + 'DROP PROC ' + QUOTENAME(s.name) + '.' + QUOTENAME(p.name) + ';
FROM sys.sql_modules sm
JOIN sys.procedures p ON sm.object_id = p.object_id
JOIN sys.schemas s ON p.schema_id = s.schema_id
WHERE sm.definition NOT LIKE '%string your looking for%'
AND p.is_ms_shipped = 0;
-- uncomment when ready to do work
3. Script all remaining procs as ALTER using the Object Explorer (Press F7 in SSMS).
edit: add "AND p.is_ms_shipped = 0" to query
__________________________________________________________________________________________________There are no special teachers of virtue, because virtue is taught by the whole community. --Plato