How can a cryptocurrency demotivate investment in favour of continued transactions? Is it economically and practically possible to create a system where investment cannot take place?(or if it does, it doesn't affect the system in a big way like Bitcoin had extremely high prices due to low supply and high demand)

What I thought of is a system where after a certain threshold(coins*time_left_inactive) the coins return to the system as a reward to every miner proportional to the hashing power they provide the system with.

But this has a problem, in order to bypass said limitation one can just continue transactions between two(or more) wallets and miners would be mining fake(I couldn't think of a better term here) transactions, greatly increasing the hashing power the system requires unnecessarily. 

I don't know of any existing currencies that tackle said problem(Obviously)
So if there are any, please let me know, I'd like to look into 'em.
You could try demurrage, where the value of one's holdings is intentionally reduced periodically. Freicoin is the only example I know of in the crypto space.