He probably intended to use a memory spell
Dumbledore probably would've manipulated Kreacher's memories: either wiping out all the elf knew about the Order, or convincing him that he belonged to Harry or to Hogwarts. Maybe even made him forget his own name and the Black family. There is no canon information how memory charms affect the bond between the elf and his owner, but if Kreacher forgot his own name, there is a good chance his mistress, Bellatrix couldn't have summoned him.
Dumbledore is not above using spells to affect the perception of those who would not submit to his will: he used a spell to confound the manager of Wool's orphanage, Mrs Cole:
There was no doubt that Mrs. Cole was an inconveniently sharp woman.
Apparently Dumbledore thought so too, for Harry now saw him slip his
wand out of the pocket of his velvet suit, at the same time picking up
a piece of perfectly blank paper from Mrs. Cole’s desktop. “Here,”
said Dumbledore, waving his wand once as he passed her the piece of
paper, “I think this will make everything clear.” Mrs. Cole’s eyes
slid out of focus and back again as she gazed intently at the blank
paper for a moment. “That seems perfectly in order,” she said
placidly, handing it back. ~Harry Potter and the Half Blood Prince - The Secret Riddle
also when Dumbledore was collecting information about Tom Riddle, when he extracted memories it wasn't always with the full cooperation and approval of the subject. He had no scruples to invade someone's brain if needed, I don't imagine Morphin Gaunt, for one, was very cooperative:
“But he had this real memory in him all the time!” “Yes, but it took a
great deal of skilled Legilimency to coax it out of him,” said
Dumbledore, “and why should anybody delve further into Morfin’s mind
when he had already confessed to the crime? However, I was able to
secure a visit to Morfin in the last weeks of his life, by which time
I was attempting to discover as much as I could about Voldemort’s
past. I extracted this memory with difficulty. ~Harry Potter and the Half Blood Prince - A Sluggish Memory
Memory charms frequently used in the wizarding world, they even have an occupation called 'obliviator': professionals used by the Ministry of Magic to modify memories. Dumbledore, if necessary, would have used a spell to obliviate or confound Kreacher to protect the Order, presumably preferring this solution to imprisoning Kreatcher - which he also could have done. House elf magic is different, they can do wandless magic and are not affected by the usual anti-apparation wards, but it doesn't mean they can't be affected by magic. It's true that Kreatcher escaped from Voldemort's cave, but it's not because the magic of house elves is unbeatable by wizards, rather because Voldemort thought house elves were negligible - the possibility of Regulus Black's house elf escaping hadn't even occurred to him. Dumbledore, on the other hand wouldn't do such a mistake - he was well aware of the consequences of Kreacher joining his mistress, so he'd have taken the necessary measures if needed.
We know house elves are susceptible to mind spells, after all they were subjugated and mentally bound by wizards. A brilliant wizard like Dumbledore, who was also proficient in legilimency and occlumency would have found a solution to affect the mind of Kreatcher if he had no other choice.
He could've convinced Kreacher that his life's ambition was to move to Australia and open a kangaroo farm or any other place out range of the slave-master connection.