The most straightforward answer I can give you is: have them gain less treasure during the next encounters, until you reach the intended balance.

In games where XP and money (or their equivalents) are independent, it's fairly easy. The PCs are now over-equipped, so they could also be able to handle some higher level fights for bigger XP awards. While accelerating the XP gain is not necessary, it could solve your problem and let you all go back to the regular encounters faster.  
Choose enemies that don't need any or much equipment to avoid having yet more wealth land in the PCs' hands. Have their level catch up to wealth. If you want it to resolve even faster, you can increase the XP rewards.

Keep in mind that if you increased the encounter level (assuming the game cares about such things) the PCs will level up faster than normal while you are fixing the balance. Be careful if this causes pacing problems for your campaign or your players.

This is best done by dropping some items now and then (thus slowing your catching-up process), since stopping treasure gain for a long time is not fun.