How should I avoid someone patenting technology in my paper/poster?
You need not patent your invention. The requirements of a patent are that it is new, useful and non-obvious. To another person your published work is considered prior art. If someone else tried to patent your work the patent office would reject it as anticipated. 

Of course this assumes the patent office finds your work. The inventor could fail to disclose it and the patent office could fail to find it in search. In such a case you can typically submit your work to the relavent patent offices to have the patent/patent application invalidated/rejected.