Aphids are always with us - they're flying insects and can affect indoor plants (especially in summer) as well as outdoor ones. Using a soapy water spray may well kill any which are currently present, but will not prevent more arriving. As this is not a food plant (meaning you're not going to be eating any part of it) you can use a systemic insecticide. Insecticides are available in two types - contact or systemic. Contact is similar to your soapy water spray, kills what you get the spray on, but systemic ones not only kill on contact, they enter the sapstream of the plant and prevent further infestations for up to two weeks, depending on the product.
If you keep having trouble with aphids, it's probably best to invest in a bottle of systemic insecticide available in your area, and apply according to the instructions. If you can take the plant outside to spray it, do so, and when you spray ensure you cover the entire plant, under and on top of the leaves, and the stems, till run off, avoiding any open flowers if at all possible.