Okay. found a solution. Giving details below, if anyone stumbles here.
The "WooCommerce Currency Switcher" plugin changes the shop currency
& stores using session / transient method. Not just that, it also adds a body class for each selected currency.
e.g.: currency-usd
or currency-eur
or currency-gbp
This is what is possible simply using CSS
.currency-usd .payment_method_instamojo, .currency-usd .payment_method_paynimo, .currency-eur .payment_method_instamojo, .currency-eur .payment_method_paynimo, .currency-gbp .payment_method_instamojo, .currency-gbp .payment_method_paynimo {display:none}
This works as expected w/o writing a separate function. Hope this helps.