Exclude databases from automatic full backup
Its easy using T-SQL.

I would suggest not to use Maiantenance plan. Instead go with a much flexible and widely adapted TSQL based solution - SQL Server Backup by Ola Hallengren 

EXECUTE dbo.DatabaseBackup
@Databases = 'USER_DATABASES, -DB1,DB2', -- All user databases, except Db1, DB2
@Directory = 'C:\Backup',
@BackupType = 'FULL',
@Verify = 'Y',
@Compress = 'Y',
@CheckSum = 'Y',
@CleanupTime = 24