Starter Solenoid staying on after start up??

[soulfly71]

hi there good ppl, fairly straight forward this I think: accidentally attached an earth to positive on the starter and nearly killed the van after doing the Head Seals on the Engine, Starter now intermittently stays on after start up, I can stop this by switching the engine off then pulling the Neg lead off the battery under the seat, however, I'd rather not do this all the time LOL
The solenoid apparently isn't returning to the off position after start up so, how do I remedy it if possible without buying a new starter motor.