1. Not every item has to be available for purchase; indeed, many items shouldn't be available. If the party is pooling money to get one character stacked with items which are considerably better than he should have, then just don't make those items available. Even then, a Belt of Strength +6 takes thirty-six days to craft, and a lot can go on in-game between that. An urgent adventure may take them to a land far away from where the item was commissioned, and by the time they finally arrive back to retrieve it they may find that the creator has sold it to someone else, but he'll make a new one for them after another thirty-six days. It's partially your own fault that this has happened.

2. You said he's a Lizardfolk with Sorcerer, Fighter, and Dragon Disciple, and he's currently level 11 but casting Rage. Rage is a 3rd level spell, Lizardfolk have a +1 LA and two mandatory Humanoid racial HD, so that means that unless either he's cheating or you've made some ill-informed houserules he's gone LA +1/ Humanoid 2/ Sorcerer 6/ Fighter 1/ Dragon Disciple 1. Dragon Disciple does not grant access to any higher level spells than he already had access to. That's not even considering the extremely short duration of Rage considering his low caster level and the action requirement for concentration. If you made houserules which allowed him to do this, then again it's your own mistake that this has happened, if not then you need to audit his character sheet and make him fix anything that's not permissible.

3. Use mixed encounters, throw in a big, strong opponent which the problem character has to deal with personally, and several other opponents who are a fitting challenge for the rest of the party. Looking down the CR 8-10 list in the Monster Manual, such creatures could include a Greater Elemental, a Stone, Frost, or Fire Giant, various Gargantuan Monstrous Vermin, a Hydra, etc. If the party is one strong character and several weaker characters, then make the encounters more complicated with a strong opponent that the weaker characters won't want to engage but plenty of other opponents for them to deal with.