The League One side reportedly offered £125,000 for the 26-year-old, which is thought to be below Bournemouth's valuation of the player.
Howe told BBC Radio Solent: "Wes isn't for sale. He's part of our plans.
"[Swindon] haven't been encouraged and the bid has been rejected. We are low on strikers at the moment so it's a case of wanting to keep everyone at the moment."
Thomas has scored 11 goals in 48 appearances since joining Bournemouth in 2011.