There is the last option - don't open them. They revert to your incoming text message list, and you can look at the number it was sent from to determine whether you want to open it. Delete is just a keystroke away, no charged airtime.
At best, that works if few have your number, if 100 others are potential senders, then it has to be asked if a PAYG phone is really what you need. Another finesse point is simply chucking the thing when it runs out of minutes - you're not on a lease, Tracphones are cheap commodity items off the hook, and for some, changing the number on occasion is beneficial. Very few actually dial numbers, they select names off a list - and changing one on the list with a simple text to say "This is my new number." updates it with the people you need to have know. Not every chance acquaintance or email spammer.
Most of what I read here on TF use is based on thinking of them like a plan lease phone - they aren't by any comparison. The marketing and tech behind them is radically different in some issues, not always to the users advantage. Compare and decide. Customer Service get generally poor marks, cost is great, but the phone is chainganged to the SIM and local carrier, making it difficult to alter. ALL the airtime is leased from someone else, and parceled out to minimize cost, sometimes at the loss of better coverage from a better saturated carrier.
You get what you pay for.