If the spellthief can supply the spell's components and the spell's level is appropriate, the spellthief can cast a stolen spell into a minor ring of spell storing
The relevant portions of the spellthief's special ability steal spells says that
After stealing a spell, a spellthief can cast the spell himself on a subsequent turn. Treat the spell as if it were cast by the original owner of the spell for the purpose of determining caster level, save DC, and so forth. A spellthief can cast this spell even if he doesn’t have the minimum ability score normally required to cast a spell of that level. The spellthief must supply the same components (including verbal, somatic, material, XP, and any focus) required for the stolen spell.... A spellthief must cast a stolen spell (or use its energy to cast one of his own spells) within 1 hour of stealing it; otherwise, the extra spell energy fades harmlessly away. (Complete Adventurer 16)
Emphasis mine. Note that the 18,000 gp price tag on the minor ring of spell storing (and twice that for an ioun stone (vibrant purple prism) and over five times that for a major ring of spell storing) puts this tactic out of reach for low-level spellthieves. A lower-level spellthief might choose to store an appropriate spell indefinitely in a more reasonably priced +1 spell storing weapon, costing 8,000 gp and some change.
None of these receptacles appear to care how the spell arrives within them.
Further, the spell is only stored in the receptacle until the spell's cast from the receptacle. Then the spell's gone, and the spellthief's back to thieving from caster's heads.