Targeting ISIS' financial support and locating the cash used to buy arms can definitely help defeat them. Of course that is not the only goal. Every possible angle should be investigated. The money they need is just one of the several strategies that need to be pursued. People and groups that support them financially should be watched.
Generally following the money is a pretty sound strategy. If you cut off the supply source, ISIS will struggle to access funds for weapons, supplies, transportation. Lack of money it would be more difficult for them to mobilize and continue their attacks, to travel from country to country, and to buy plane, bus or train tickets or passports to escape.
While ISIS will suffer from targeting their financial assets and officers working in their financial unit, removing funding cannot be used by itself with effective results. Firstly, ISIS have a huge stockpile of weapons and supplies and can always gain more by advancing into further territory. Secondly, the means by which they accrue their funding - extortion, robbery, selling looted goods and donations from sympathisers, are methods which can be started up again quickly and efficiently if they should lose money. Therefore financial targeting should be done in conjunction with military methods otherwise they will only succeed in inconveniencing them.
Targeting and destroying monetary assets is not going to be effective in defeating ISIS. I think that this will simply make them more desperate to make money and then they will being to perform even more acts of terrorism. We need to kill their leaders and take away any hope they have for survival or accomplishing their goals in order to defeat them.